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CONTRIBUTORS

Konstantin I. Popov
Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Yugoslavia

Electrometallurgy is a broad field but it is not a new one. It was the great Faraday
in the 1830s who discovered laws covering the electrodeposition of metals and its
relation to the current passed and equivalent weight of the metal undergoing deposition. Since that time, applications and developments of his discoveries have spread to
many areas of technology. Electrowinning is the most well known, partly because it
embraces the process by which aluminum is extracted from its ores. In electrorefining,
the impure metal is made into anode and the pure metal dissolved therefrom is
deposited on a cathode. Electroplating is exemplified by its use in the manufacture of
car bumpers. Finally, in electroreforming, objects may be metallized, often with a very
thin layer of the coating desired.
The numerous technologies vary greatly in the degree to which they are intellectualized. Until the work of Popov et al., electrometallurgy has been regarded as largely
empirical, an activity in which there was much art and little science. This will all
change with the publication of this book. Several aspects of the background of its
senior author, Konstantin Popov, make him uniquely suited to the job of intellectualizing electrometallurgy. First, he had as his mentor the great
surely
the leading electrochemist in Eastern Europe since the death of Frumkin. Second, he
has had ample experience with the leading electrochemical engineer in America,
Ralph White. And third, he has an admirable track record of a series of publications
aimed at showing the remarkable variety of forms which may be made to arise in
electrodeposition.
Dr. Popovâ&#x20AC;&#x2122;s contributions are characterized by a comprehensive mathematical
treatment of the phenomena he has discovered. Co-author
too, has had
much relevant experience in applying the ideas so extensively developed in Belgrade
at Canadian companies.
The contents of the present volume illustrate how certain fields of science become
settled in certain countries. Industrial organic chemistry grew up in Germany, and
developed predominantly there until World War II. In electrometallurgy, the intellectual development has been largely in Serbia, with the most fundamental studies in
nucleation being carried out in the neighboring country of Bulgaria (Kaischev, Busevski). This is not to imply that there have been few American contributors (one recalls
Brenner, Kardos, Lowenheim), but that the contributions made here in this field have

vii

viii

Foreword

come from commercial laboratories and have been near the commercialization stage.
University or research institute work on electrometallurgical topics in the United States
has been nonexistent since the 1950s.
There are many figures in this splendid book of Popov et al. which impress me.
The first is the strong, broad contents of its arrangement. There is a fine first chapter on
the principles of application to electrochemical kinetics—the equations being written
in a form modified for use in electrometallurgical situations (e.g., deposition on the tips
of growing crystals of minimal radius of curvature and on corners and edges). Here, it
is encouraging to find authors applying the electrochemical version of Kelvin’s equation relating vapor pressure as a function of the radius of drops to the phenomena
during the electrogrowth of dendrites.
I personally find the treatments of the effects of current varying regimes (e.g.,
pulse, reverse pulse, square wave, sinusoidal, etc.) the most exciting for I have long
thought that instead of the use of chemical additives to the solution, the type of surface
finally produced—even the crystal shape—could be achieved by electrical variations
only. This book contains much toward the realization of this approach.
In the second half of the book, one finds the mathematical treatments of practical
situations in electrowinning, electrorefining, electroplating, and electroreforming.
What is the difference all this will make? It should enable to engineer to set up regimes
to achieve what he wants with a minimum of prefatory experiments.
This book has no competitor. There are certainly books on electroplating, but
they are largely recipes for what to do which eschew the important question of why.
Getting the intellectual side over to the practical engineer, of course, requires
great lucidity, for he will not puzzle over material delivered over his head. I think the
required clarity has been attained herewith, particularly in the early chapters where
the concepts of exchange currents and overpotential are being added to the weary
thermodynamics which covers most of what engineers are likely to know about
electrochemistry.
A great strength is in the photographs of electrodeposited crystals in all their
variety. Such photographs can be found in the usual journals, but I have not previously
seen such a collection accompanied by textual rationalization.
Lastly, I was impressed by the application of the theory to areas which normally
receive little more than a definition. I would cite electropolishing, where theory is
seldom presented; electromachining; and electroless plating.
This book is a feast simply to read, but I believe its main importance is that it gives
for the first time an educational tool. It will surely lead to translations and its use will
feed back upon the economics of electrometallurgical processes—with a reduction in
cost many orders of magnitude greater than the total in purchases of the books.
John O’M. Bockris
Texas A&M University

PREFACE

In their preface to the book Fundamental Aspects of Electrocrystallization,
Bockris and Razumney in 1966 wrote: “Electroplating, the electrochemical extraction
of metals from ores and mixtures, and electrodeposition from nonaqueous solutions,
together make up a large area in technology. It is relevant to distinguish two classes of
technologies: those in which, the fundamentals and theory were understood first and
the applications followed, and those in which the applied side began as a kind of art and
the theory limped behind the art, sometimes dragged back by it and occasionally given
a little push. Of course, the atomic energy and electronic industry are ideal example of
the first two. The electrodeposition and electroextraction industry is a fair example of
the second. It is clear enough that the rate of development of the first type of industry
was much greater than that of the second, as the possibilities could be estimated or at
least the direction in which to push defined, whereas the second type relied in the past
on what might be called “inspired groping.” It is hoped that this small monograph may
provide a basis for the training of research workers who can then perform the conversion of what remains an art in technological electrocrystallization into a technology
with a largely rational basis.” In the meantime, although the electrochemistry of metals
has significantly advanced, the message of Bockris and Razumney, especially in the
light of education professionals, is practically the same. The bridge between high level
theory and highly developed practice is still necessary, probably even more than
previously. The purpose of this book is to enable this. It is written in order to explain
the principles of electrometallurgical technologies, not to give their technical details. It
should be easy to follow by a reader with a graduate or an undergraduate degree in
either engineering or science. It assumes the knowledge of the basic facts required on
the level of Modern Electrochemistry by Bockris and Reddy.
The book comprises twelve chapters, which can tentatively be divided into three
parts: Chapters 1,2,6, and 7 are written by Grgur and Popov, Chapters 3–5 and 8 by
Popov and Grgur and Chapters 9–12 by
In the first part (Chapters 1 and 2) the significance of electrometallurgy in science
and technology is discussed. The fundamentals of electrochemistry necessary for an
understanding of electrometallurgical processes are also given. In the second part
(Chapters 3–6) the mechanisms of metal deposition are discussed at a high scientific
level although efforts were made to simplify them in an approachable way. For

ix

x

Preface

example the text related to the morphology of electrodeposited metals is separated into
three levels: the mathematical approach, the physical model and, finally, a realistic
system. The current distribution in electrochemical cells is described in a new way.
Using this approach, classical complicated mathematical methods are not only
avoided, but also calculations of typical examples for electrochemical cell are performed using simple relations of electrochemical kinetics (described in Chapter 2).
The reader is also introduced to the effects of additives and periodically changing rate
at a level which allows an understanding of metal deposition under these conditions. In
addition, in the third part (Chapters 6â&#x20AC;&#x201C;12), theoretical aspects of the electrowinning
and electrorefining of metals, as well as electro- and electroless deposition of thin films
used in electronics, automotive and aerospace applications are discussed. Electrodeposition of thin films of metals, alloys or composite coatings under direct or
periodically changing current conditions from aqueous or non-aqueous solutions, as
well as molten salts is described. Attempts have been made to up-to-date the current
knowledge with kinetics and mechanisms of electroless deposition and to critically
evaluate the similarities and differences of deposition processes with and without an
external current source.
In the next stage of technological development, we hope, this book will initiate
efforts for the advancement of theoretical fundamentals of electrometallurgy at a level,
which will permit the planning of technological processes without or with a minimum
of experimental data. This is in a way similar to challenge of Bockris and Razumney, a
half-century ago, which led to the appearance of this book.

ACKNOWLEDGMENTS

Chapters 1â&#x20AC;&#x201C;8 of this book are based on a few classical studies and on research in
the field of metal electrodeposition performed in the Department of Physical Chemistry and Electrochemistry at the Faculty of Technology and Metallurgy, the University
of Belgrade, Yugoslavia. K. I. Popov and B. N. Grgur would like to acknowledge
Professor
who initiated this research, and colleagues Professors
and
as well as numerous other colleagues and students who participate
in it.
Chapters 9 to 12 represent a discussion of various aspects of electrodeposition of
alloys, composite coatings, electroforming, electrochemical oxidation of metals, electroless deposition and electrodeposition from molten salts, found in the open literature.
acknowledges the cooperation of numerous colleagues and coworkers from
Sherritt Inc., The Westaim Corporation, and Dr. B. E. Conway at the University of
Ottawa. For the help in the literature search and manuscript preparation,
is
thankful to
and Mrs. C. Lepard.
Finally, we wish to express our thanks to Mr. Ken Howell for helpful advice and
to
for the preparation of the camera-ready manuscript.

Equal Plane Parallel Electrodes Arrangement
Ohmic resistance of the cell
The very edge ohmic resistance
The edge effect
The depth of the penetration of a current line between
the electrode edges and the cell side walls
4.1.4.1 Mathematical model
4.1.4.2 Cell voltage-current density dependencies
4.1.4.3 Determination of the current density distribution
4.1.5 Quantitative treatment
4.1.5.1 Calculation of the cell voltage-current density
distribution dependences
4.1.5.2 The critical current density for dendritic growth
initiation at the edges
Cells with Low Anode Polarisation
4.2.1 The dependence of the current density at the tip of a
stationary wire electrode on the current density in the middle
of the electrode
4.2.2 Experimental evidence
4.2.2.1 The effect of ohmic resistance
4.2.2.2 Deposition in the presence of strongly adsorbed
organic additives (effect of increased cathodic Tafel
slope)
4.2.2.3 Deposition from a complex salt solution (effect of
exchange current density)
Corner Weakness Phenomena in Electroforming
4.3.1 Ohmic controlled deposition
4.3.2 Mixed activation - diffusion - ohmic controlled deposition
4.3.3 Activation - diffusion controlled deposition
Conclusions
Further Readings

5. ELECTRODEPOSITION AT A PERIODICALLY CHANGING
RATE
5.1 Basic Definitions
5.1.1 Reversing current
5.1.2 Pulsating current
5.1.3 Alternating current superimposed on direct current

xv
101
103
103
106
108
109
109
111
115
118
119
122
124

124
127
127

129
131
133
133
136
140
141
142

145
145
145
146
148

xvi

Contents
5.1.4 Pulsating overpotential
5.2 Surface Concentration of Depositing Ions in the Periodic
Condition
5.2.1 Electrodeposition with periodically changing rate in the
millisecond range
5.2.2 Capacitance effects
5.2.3 Reversing current in the second range
5.3 Prevention of the Formation of Spongy Deposits and the Effect on
Dendritic Particles
5.4 Compact Deposits
5.4.1 Surface film
5.4.2 Electrode surface coarsening
5.5 Current Density and Morphology Distribution on a Macroprofile
5.6 Conclusions
5.7 Further Readings

8. OPTIMUM CONDITIONS FOR ELECTROPLATING
8.1 Cementation and Deposition from the Complex Salt Solutions
8.2 The Porosity of Metal Electrodeposits
8.3 The Condition for the Deposition of a Coating with a Minimum
Porosity
8.4 Further Readings

Electrometallurgy deals with technical aspect of metal electrodeposition.
The electrometallurgical processes can be categorized into four main groups:
1. Electrowinning,
2. Electrorefining,
3. Electroplating,
4. Electroforming.
The schematic representation of electrometallurgical processes is shown
in Fig. 1.1.

1

2

Chapter 1

Electrowinning is the extraction of metals by electrodeposition from aqueous solution or melts of their salts. On a large
scale electrodeposition from molten salts is used for extraction
of electronegative elements which cannot be electrodeposited
from aqueous solutions, such as aluminum and magnesium, as
well as very pure copper, zinc and cadmium by electrodeposition from an aqueous solutions of the metal salts1.
Electrorefining is the purification of metals by electrolysis.
The impure metals is dissolved anodically and pure metal is
deposited catodically, while the impurities being left as anode
sludge or as ions in the solution. Many metals are electrorefined
such as copper because of conducting application and precious
metals because of theirs cost. Electrorefining is also a part of
processes in recycling of metals.
It should be noted that large electrolytic plants for metal
production are heavy consumers of electric energy1. In the
metal electrorefining and electrowinning the main requirements
are to produce pure and compact deposits. This is done at lower
current densities. From qualitatively the same, but less concentrated solutions at higher current densities metal deposits in
form of powder are obtained. Powder electrodeposition can also
be treated as kind of electrowinning or electrorefining, which
produces the metal deposits in forms suitable for sintering and
various different applications.
Electroplating can be defined as a treatment that modifies the
surface of a metal or occasionally a nonmetal, without changing
its bulk properties, in order to improve the appearance of a
surface, to increase the corrosion and abrasion resistivity, etc.
The improving the appearance was the aim of electroplating
earlier, now it is mainly the change of surface properties from
those of substrate material to those of electroplated metal.
Obviously, the coating can successfully change the surface properties of substrate only if it is compact and nonporous, as well
as good adherent1-3. Metal objects we meet in everyday life are
often electroplated, but it seams that the most important
application of electroplating technology is the manufacture of
electronic components (circuit breakers and contacts). Electroplating can be performed from molten salts and aqueous and
non-aquaeous solutions, depending on the nature of electrodeposited metal, but most frequently from aqueous solutions1-3.
Electroforming is the manufacture of articles by electrodeposition. If deposit is good from electroplating point of view

1. What is Electrometallurgy

3

except adhesion, and can be removed from the cathode as an
entity in itself, it has been electroformed. Electroforming is a
branch of electroplating technology, but involve some additional steps, as for example the production, preparation and
extraction of the master2,4.
Electroless metal deposition and anodic oxidation of metals can also be
include in the field of electrometallurgy.
Empirically is known what type of deposit can be obtained under specific
conditions, however how and why this can be achieved still remains a
mystery in some cases.
The aim of this book is to give answers to some of open questions.

In a metallic conductor free conduction electrons transport the charge
whereas in an electrolytic conductor it is ions. In order to include an
electrolytic conductor in an electrochemical circuit it is necessary to make
electrical contacts to and from the electrolyte by metallic conductors. A
metallic conductor immersed in an electrolyte solution is an electrode, and
two electrodes connected electrolytically represent an electrochemical cell1,2.
The simplest electrochemical circuit is shown in Fig. 2.1.

5

6

Chapter 2

The electrochemical circuit consists of a current source, metallic connecting wires, an electrochemical cell, ohmic resistance, current and voltage
measuring instruments and a circuit breaker. In technical practice more
complicated circuits are used, but in principle all of them are the same as the
one shown in Fig. 2.1.
Obviously, a steady current flow in the circuit from Fig. 2.1 can only be
maintained if there is a change of charge carrier at the metal-electrolyte
interface by a chemical transformation involving the transfer of electrons
across the interface, i.e., by an electrochemical reaction. It constitutes the
bridge between the current of electrons in the metallic part of the
electrochemical circuit and the current of ions in the electrolytic part of the
circuit1,2.
2.1.2

If the metal ions in the solution are the same as in the electrode metal
lattice, or if the same substance is present in the electrolyte in two oxidation
states, an electron transfer reaction can occur at the metal-electrolyte
interface and lead to the development of a potential difference. Such an
interface behaves like an electrical circuit consisting of a resistor and a
capacitor in parallel. The electron transfer takes place until a dynamic
equilibrium is reached. In the case of metal electrodes, depending on the
system, this process begins with either the deposition of ions from solution
onto the metal electrode or with the dissolution of the metal electrode. In
equilibrium the electrode is more positive than the solution in the first case
and more negative in the second one. A number of electrochemical reactions
are possible at such an interface, as for example
1. The reduction of metal cations to the metal and vice versa

2. The reduction of hydrogen cations to gaseous hydrogen and vice verse

3. The decrease of the oxidation state of the cations and vice versa

2. Definitions, Principles and Concepts

7

4. The reduction of anions to metal and vice versa

5. The reduction of molecules to anions and vice versa
(in alkaline media)
6. The oxidation of molecules to cations and vice versa
(in acid media)

2.2

SELF DRIVING CELLS

2.2.1

The Nernst equation and energy producing cells

For the electrochemical reaction

where O is the oxidized state which accepts n electrons and R is the
reduction state or the donor of electrons, in equilibrium, the Nernst equation3
is written in the form

where
is the equilibrium electrode potential,
is the standard
electrode potential and
and
are the activities of the electron acceptor
and donor, respectively1,2.
In Table 2.1. the standard potentials for some electrode reactions are
given.

8

Chapter 2

The signs of two electrodes connected to a cell can be determined by
using the values of standard electrode potentials. Only, if they are close to
each other the Nernst equation should be used to determine the polarity of
them.
The equilibrium potential difference for the cell

which is illustrated in Fig. 2.2, can be evaluated as follows.

Obviously, the equilibrium concentration of electrons in the more
negative electrode will be larger than in the more positive one.
According to Table 2.1, the possible reactions on the electrodes are:

and

2. Definitions, Principles and Concepts

9

The equilibrium potential difference E is given by

If the electrodes are connected as in Fig. 2.3, the reaction on the
electrodes are:

and

The overall reaction in is then:

Obviously, oxidation will take place on the more negative electrode,
making it an electron sink, and reduction will occur on the more positive
electrode, making it an electron source.
Such electrochemical transformations at the two interfaces provides a
steam of electrons available for external use, which is the essence of energy
producing cells (electrochemical power sources).

10

Chapter 2

Reaction, given by Eq. 2.8 will go from left to right until the potentials of
the electrodes become equal, which corresponds to the zero cell voltage and
equilibrium activities
and
of zinc and copper ions, respectively. It follows then from Eq. 2.5 that if E = 0

where

represents the equilibrium constant for reaction 2.8. The value of
means that the
ions can be completely removed from the electrolytic
solution by reaction 2.8.
2.2.2

Cementation

If a piece of zinc is immersed in a copper sulfate solution the reaction 2.6
occur at one local area of the electronic conductor and reaction 2.7 at another
local area. Reaction 2.6 and 2.7 occur spontaneously, and the overall
reaction is given by Eq. 2.8. The system is self-driven and produces power,
but the corresponding electrochemical energy is unavailable because the two
interfaces are short-circuited3. This reaction is used in purification of zinc
sulphate solution from more positive metallic ion impurities in zinc
electrowinning process.

2.3

ELECTROLYSIS

2.3.1

Decomposition voltage

The self-driving cell from Fig. 2.3 can be rearranged to a driven cell by
connecting the power supply in the circuit as shown in Fig. 2.43.
The reactions on the electrodes will be

and

2. Definitions, Principles and Concepts

11

and the electron steam will flow in the opposite direction to that in the same
cell working as a self driven one only if the power supply voltage is larger
than the equilibrium potential difference of the same cell, working as a self
driven one.

If two or more anodic and cathodic reactions are possible in some driven
cell, the reactions with the lowest equilibrium potential difference will take
place on the electrodes first. This means that the reaction with the most
positive equilibrium potential will take place first on the cathode (the
electrode connected with the negative terminal of power supply, at which
reduction occurs), and the reaction with the most negative equilibrium
potential on the anode (the electrode connected with the positive terminal of
the power supply, at which oxidation occurs). It is important to remember
that the terms anode and cathode are connected with the nature of the
reaction (oxidation or reduction) at the electrode and not with their polarity.
Thus, in a self driven cell, the anode is the negative terminal and the cathode
is the positive terminal of the cell, a situation which is precisely the opposite
of that which exists in an externally driven cell3.
2.3.2

A cell with an insoluble anode

In the driven cell

the following reactions on the anode (Au) are possible:

12

Chapter 2

and on the cathode (Cu)

According to the rule derived in the conclusion from section 2.3.1, on the
anode the oxygen evolution reaction will occur, and on the cathode copper
ions will be reduced to the metal phase, because the most positive catodic
reaction can be neglected due to the low oxygen concentration in the
electrolyte solution. Hence the reaction:

will take place at the anode and the reaction

at the cathode.
Obviously, the minimum external cell voltage for electrolysis to occur in
this case is 0.893 V.
The overall reaction in the cell is

It follows from the Eq. 2.14 that in a cell with an insoluble anode the
concentration of depositing ions decreases and the hydrogen ion concentration increases during electrolysis.

2. Definitions, Principles and Concepts

13

The mechanism of the extraction of metals from ionic solutions and the
essence of the electrowinning process are well explained by Eq. 2.14.
2.3.3

A cell with a soluble anode

In the cell:

the following reactions are possible on the anode (Cu):

and on the cathode (Au):

Hence, the reaction

occurs on the anode, and

at the cathode, and so the composition of the electrolyte solution remains
constant if the anode is made of pure copper and oxygen is removed from
the solution. The lowest cell voltage at which electrolysis can start in this
cell is zero.

14

Chapter 2

It is obvious that electrorefining processes are based on electrolysis in
cells with soluble anodes.
2.3.4

Current efficiency

Metal deposition can be accompanied by any other cathodic reaction,
most frequently hydrogen evolution.
This leads to the situation in which metal is deposited but metal
deposition uses only a part,
of the total current, I, through the cell. The
current efficiency

indicates which part of the total current is used for the deposition of metals.
It is a very important parameter of an electrodeposition process3.
2.3.5

Faraday’s law

Faraday’s law relates the quantity of electricity passed through the cell
and the quantity of chemical substances which react on the electrodes. It
states that the mass of metal, m, electrodeposited on the cathode is given by

where / is the total current, t is the deposition time, M is the molar mass of
the deposited metal,
is the current efficiency and nF is the number of
Faradays per mole of consumed ions. It follows from Eq. 2.16 that can be
easily determined by measuring the electrodeposited mass of metals and
supplied quantity of electricity1,2.
2.3.6

The current density – overpotential relationships

2.3.6.1 Basic equations
The most complete discussion of the current density – overpotential
relationship was given by Bockris4. The approach suitable for use in metal
electrodeposition will be given here.
The general form of current density – overpotential relationship in
electrodeposition of metals for the reaction

2. Definitions, Principles and Concepts

15

taking cathodic current density and overpotential as positive, is given by

where is the exchange current density and is the activity of the oxidized
(O) or reduced (R) state at a current density j and
is the activity in the
equilibrium state.5
On the other hand

and

where
and
are the cathodic and anodic transfer coefficient,
corresponding Tafel slopes and is the overpotential and

and

and

The ratio of the activities for the cathodic reaction may be written as:

where is the limiting diffusion current density and for the reverse anodic
reaction as:5

16

Chapter 2

taking into account the Kelvin term which becomes appreciable at low values
of electrode radii6. In Eq. 2.23 is the surface energy, V is the molar volume of
the electrodeposited metal and
is the radius of the electrode. Eq. 2.23 is
5
valid for two electron reactions , other possibilities are discussed in Ref. 7.
For a spherical electrode Eq. 2.17 can be written as:

or

where

where
is the bulk concentration and D diffusivity coefficient of a
depositing ions.
A somewhat modified Eq 2.25 is necessary for an understanding of
electrodeposition on the tip of dendrites inside the diffusion layer of a
macroelectrode, especially in the case of electrodeposition at a periodically
changing rate7-9.
For sufficiently large to make surface energy term negligible Eq. 2.25
can be rewritten in the form:

For a sheet electrodes and sufficiently large spherical electrodes Eq. 2.25
becomes:

2. Definitions, Principles and Concepts

17

where

and

is the diffusion layer thickness3.

2.3.6.2 Some approximations
Equation 2.27 is valid generally but it is more convenient to use some
approximative relations derived from them, so for flat surface if

Eq. 2.28 can be rewritten in the form:

which becomes

at very low overpotentials by expanding the exponential terms in Eq. 2.31
and retaining the two terms of expansion of each exponential terms, if

and

where is a the symmetry factor.
When

the relation:

18

Chapter 2

or

is valid.
If

Eq. 2.28 becomes:

or

For

Eq. 2.28 can be rewritten in the form:

which is valid if Eqs. 2.33 and 2.34 are valid and finally, if

Eq. 2.39 becomes

The range of validity of Eqs. 2.31 and 2.32, 2.36 and 2.37 and 2.38 and
2.39 and 2.43 can be easily determined from
and
plots. The
equations 2.36 and 2.37 are valid from the beginning to the end of Tafel
linearity (Tafel line). At lower overpotentials, Eqs. 2.31, and 2.32 are valid

2. Definitions, Principles and Concepts

19

and at higher ones, Eq. 2.38, 2.39 and 2.43. In the Fig. 2.5 the simulated
polarization curve for cathodic metal electrodeposition together with the
Tafel plot and the range of validity of mentioned equations are shown.
Usually, in electrochemistry the marked regions are called: 1 and 2activation controlled region, 3- mixed activation-diffusion controlled region,
4- pure diffusion controlled region.

2.3.7

The cell voltage

It is to be noted that the difference of cathodic and anodic overpotentials
is the sum of their absolute values. Hence, if anode overpotential is
considered in the anode Tafel region the absolute values of overpotentrials
and current density should be used, and equation

is valid.
There are not mass transport limitations in anodic dissolution of metals
and there is not equation analoguous to Eq. 2.39.

20

Chapter 2

It is now possible to define the cell potential in electrolysis. The cell
voltage U of a driven electrochemical cell is given by:

where E is the equilibrium potential difference between the anode and the
cathode,
and
are the absolute values of the anodic and cathodic overpotentials respectively, I is the current, and

is the sum of the Ohmic

resistance of the electrolyte, electrodes, contacts and connecting wires.
2.3.8

Specific energy consumption

The electrical work W required to deposit a quantity m of metal on the
cathode is given by:

On the other hand, the quantity of deposited metal and the required
quantity of electricity for deposition are related by the equation:

By combining Eq. 2.16 with Eq. 2.46, the specific energy consumption,
w, is then given by:

The specific energy consumption, w, is the most important energetic
parameter in metal electrowinning and electrorefining technologies.

2.4

SOME ASPECTS OF
ELECTROCRYSTALLIZATION

Figure 2.6 shows the originally published in-situ STM images of copper
clusters which were formed during electrodeposition on Au(111) substrate10.
The top image shows the gold substrate at a potential positive of the
Nernst potential for bulk copper deposition. This area of the surface is

2. Definitions, Principles and Concepts

21

characterised by two atomically smooth terraces separated by a monoatomic
high step edge. Upon stepping the electrode potential to a value negative of
the Nernst potential, distinctive copper clusters form at the step edges. By
contrast, the terraces in Fig 2.6b remain free from copper clusters. The
clusters are seen to grow from Fig. 2.6b to the subsequent image 2.6c. These
STM images are a visual verification of the important role which defects can
play in the initial stages of electrocrystallization. Clearly, they are in good
accordance with the textbook model of Kossel and Stranski3,10. Nucleation
occurs preferentially at the step edges, where an ad-atom is more high
coordinated with the surface than an ad-atom on the atomically flat surfaces.
The energetics of copper nucleation on flat gold terraces are clearly less
favorable, occurring only at longer times or higher overpotentials10.

Hence, the monoatomic high step edges, the microsteps, are required for
continuos metal electrocrystalization. Possible sources of microsteps on a surface are shown in Figs. 2.7 and 2.8, i.e. the low-index planes, two dimensional
nuclei, emergent screw dislocations and indestructible reentrant groves.7,11

22

Chapter 2

It is obvious from Figs. 2.7c and 2.7a that after the formation of a low index
plane, two-dimensional nucleation is necessary for the growth to be continued.

In the case of a reentrant grove, Fig. 2.8, the growth of new layers can be
started by one-dimensional nucleation. In the case of a screw dislocation, the
step provokes the growth by retaking itself with one end fixed at the point
where the screw dislocation emerges.
If a crystal plane lacks steps and kinks, i.e., points of growth, or if growth
at these sites is sufficiently inhibited so that a large concentration of ad-ions
builds up compared to the equilibrium concentration, the probability
increases that new growing centers will form as two-dimensional nuclei. A
very convincing illustration of such a situation was made by Budevski et
al.12 after a remarkable achievement of preparing metal surfaces free of any
dislocations. At constant current, polarizations much larger than those upon

2. Definitions, Principles and Concepts

23

ordinary planes of such metals were obtained12. Moreover, they were distinguished by periodic oscillations. These phenomena are ascribed to fluctuations in the formation of two-dimensional nuclei. Under potentiostatic conditions, a current can be observed on a dislocation-free surface only at overpotentials exceeding 8 to 12 mV, whereas the cell is electrically cut off at
lower overpotentials. When a short voltage pulse in excess of this value is
applied to a cell in the cut off condition, supersaturation by ad-ions is
achieved and a nucleus of a new lattice net is formed. The propagation of the
produced step is accompanied by a certain current flow. When the new layer
has spread out over the whole surface the current again drops to zero, since
the steady-state potential is insufficient to form a new nucleus13.

The current-time curves for a series of successive voltage pulses vary in
form, but the integral of the current over time has the same value in all cases.
The amount of electricity given by this integral corresponds exactly to the
amount required for the completion of a monoatomic layer over the cubic
plane11,13.
The growth rates in the cases of one-dimensional and two-dimensional
nucleation as rate determining steps can be compared to each other by considering the growth of two dimensional flat cadmium dendrites from Fig 2.9.
The tip of the twined cadmium dendrite precursor from Fig. 2.9a
represents the physical equivalent of the scheme of the growth site from Fig.
2.8. As shown in Fig. 2.8 an layer of atoms advance in the direction determined by twining laws, an edge is constantly renewed, in which the new

24

Chapter 2

layers can be started by one-dimensional nucleation. Further growth and
branching of precursor like that from Fig. 2.9a produces the dendrites shown
in Figs. 2.9b and 2.9c. The deposition on the lateral flat dendrite surfaces
takes place by repeated two-dimensional nucleation, as in earlier described
deposition on dislocation free surface. This makes the deposition rate in the
direction of tip motion many times larger, which results in dendrite shape
like that from Fig. 2.9c.
Let it be supposed now, that an advancing microstep suddenly stops
advancing. The movement may cease, e.g., owing to the adsorption of
impurities from the solution at the step. On a solid surface with its hierarchy
of sites, there will be a hierarchy of free energies of adsorption and it may
occur that impurities seek adsorption at steps in preference to adsorption on
flat planes.

So think of a microstep which, for a reason such that given above, has
stopped advancing somewhere within the boundaries of the crystal (Fig.
2.10). Now imagine that a layer B of atoms is growing on top of the layer A.
The step B will keep advancing until it comes to the point where the advance
of step A was blocked. The layer B will then act as though it has reached the
edge of the crystal. If the same process is repeated with another layer C on
top of layer B, and then another layer on top of layer C, and so on, then there
is a pile-up of layer upon layer. Microsteps bunch into macrosteps and
sometimes the pile-up reaches such proportions that it can be seen under a
microscope as a macrostep.3

2. Definitions, Principles and Concepts

25

The macrosteps can be clearly seen on the lateral surfaces of flat
cadmium dendrites as shown in Fig. 2.11.

The flat dendrite lateral surfaces behaves as monocrystals ones and the
questions arises: How valid is the picture of deposition, developed above,
when the electrodes are polycrystalline metals? The answer is simple. One

26

Chapter 2

can consider the surface exposed to the solution by each grain as a singlecrystal macrosubstrates. That is, one would have charge transfer followed by
surface diffusion, transfer to steps, then to kinks. etc., and one would also
have rotating addition, however steps, resulting from screw dislocation,
growth spirals, faceting, etc. In addition, however, at the grain boundaries
where the single-crystal microsubstrates meet and the periodic atomic
arrangement of each grain is interrupted, the deposition and growth
processes will be abnormal. But the actual area of an electrode surface
occupied by the grain boundaries is so negligible that the abnormal processes
occurring there can be largely ignored. In conclusion, therefore, the basic
picture of deposition and growth developed for single crystals is valid as a
basis for understanding the electrogrowth of polycrystals3.

2.5

CONCLUSIONS

In this chapter some basic definitions, principles and concepts necessary
for understanding of the following chapters have been treated. Assuming that
riders have some electrochemical knowledge, for all necessary questions we
strongly recommends further reading of excellent books of J. O’M Bockris
and A. K.N. Reddy, Modern Electrochemistry.

Morphology is probably the most important property of electrodeposited
metals. It depends mainly on the kinetic parameters of the deposition process
and on the deposition overpotential or current density. The morphology of an
electrodeposited metal depends also on the deposition time until the deposit
has attained its final form. When the electrodeposition is from pure simple or
complex salt solutions, the form of the metal electrodeposit can be:
1. compact,
2. dendritic,
3. spongy, granular (grains, boulders), etc.
There are three main cases which are divided according to the exchange
current density of deposition processes.
Deposition processes which are characterized by very large exchange
current densities. Boulders are formed at lower overpotentials and dendrites at
higher ones. In the limiting case, dendrites grow at practically all overpotentials in the electrodeposition of metals with low melting points (e.g. Sn, Pb),
deposition processes which are characterized by large exchange current
densities. Spongy-deposits appear at lower and dendrites at larger
overpotentials,
deposition processes which are characterized by medium and low
exchange current densities. Compact deposits are obtained at lower and
dendritic and spongy-dendritic deposits at larger deposition overpotentials.
On the other hand, it is known that the morphology of electrodeposits can
be substantially changed if the electrodeposition is carried out in the
presence of organic or inorganic additives. For example, a smooth deposit
can be obtained in the presence of additives instead of rough one in the
absence of additives.
29

30

Chapter 3

The processes of metal electrodeposition can be categorized into three
main groups:
1. electroplating and electroforming
2. electrorefining and electrowinning, and
3. metal powder production
each of which have different requirements with respect to the physical state
of the cathodic product.
In electroplating the crystal layer is required to be fine grained, smooth,
strongly adhesive, glassy, i.e. to be easily polished or bright. In refining and
electrowinning relatively coarse grained, rough, but adhesive deposits are
required. They have to be of high purity and firm enough to endure handling
before melting and casting into shapes suitable for further processing. In
metal powder production by electrodeposition a controlled product particle
size is necessary and it is preferable if the product is only weakly adhesive.
The first kind of deposit can be obtained from solutions characterised by
low exchange current densities at overpotentials close to the end of the Tafel
linearity in the presence of different organic additives, with different effects
on the cathode deposit.
The deposits in electrorefining and electrowinning are obtained from
electrolytes characterised by medium exchange current density at
overpotentials close to the end of Tafel linearity. Powdered electrodeposits
are obtained from the same solutions, but at current densities considerably
larger than the limiting diffusion current density for the electrodeposition
process.
Spongy and granular electrodeposits appear during deposition by
processes characterised by large exchange current densities at low overpotentials. They are not often met in electrodeposition practice. Spongy zinc
can be formed during the charging cycle of same alkaline storage batteries,
and can be easily removed by electrodeposition at a periodically charging
rate, as can the whiskers which can be formed in some plating baths in the
presence of some organic additives.
The above facts are well known, but there is no complete analysis of
them so far. The main aim of this chapter is to remedy this. This analysis is
performed for electrodeposition of metals from pure simple and complex salt
solutions at a constant rate in the presence or in the absence of additives.

3.1

THIN COMPACT SURFACE METAL FILMS

The first stage of metal electrodeposition on an inert substrate is the
formation of a thin surface film of deposited metal. It isolates the initial
substrate from the solution, and the aim of this section is to show how this
can be achieved using a minimum quantity of electrodeposit.

3. Surface Morphology of Metal Electrodeposits
3.1.1

31

Crystallization overpotential

The formation of the first crystals during galvanostatic metal electrodeposition on an inert substrate is sometimes accompanied by a pronounced
increase in the overpotential1,2. The dependence of the overpotential on time
in such situations is shown in Fig. 3.1.

The overpotential changes with deposition time from point b to point c
according to the equation1:

where
is the concentration of adatoms at time t and
is the
concentration of adatoms at t = 0.
On the other hand, the surface concentration of adatoms changes
according to:

32

Chapter 3

Substitution of Eq. 3.2 into Eq. 3.1 produces

which become

at the moment of nucleation.
Eq. 3.3 describes the dependence of the overpotential on the deposition
time from point b to point c. The overpotential changes due to the change of
the surface concentration of adatoms from
at the equilibrium potential to
some critical value
at the critical overpotential,
at which the new
phase is formed. Hence, the concentration of adatoms increases above the
equilibrium concentration during the cathode reaction, meaning that at
potentials from point b to point c there is some supersaturation. The
concentration of adatoms increases to the extent to which the boundary of
the equilibrium existence of adatoms and crystals has been assumed to
enable the formation of crystal nuclei. On the other hand, the polarisation
curve can be expressed by the equation of the charge transfer reaction,
modified with respect to the crystallisation process, if diffusion and the
reaction overpotential are negligible, that is by2:

because the partial anode current density depends on the concentration of
adatoms, which for
becomes equal to Eq. 3.3a.
Obviously, Eq. 3.4 becomes valid at the moment of the formation of the
new phase, and it can be used for the estimating the overpotential,
at
which the nucleation takes place. In order to calculate this overpotential, the
supersaturation must be known. According to Pangarov and coworkers3-5, the
work of formation of differently oriented particles can be estimated using

3. Surface Morphology of Metal Electrodeposits

33

supersaturations of 4-7. Considering the nucleation overpotential (for
different supersaturations), Klapka2 assumed 10 as the upper limit of
supersaturation. The lower limit is obviously 1 and Eq. 3.4 in this case
becomes identical to the equation of the charge transfer reaction.
The difference in overpotential between the curves for a given
supersaturation (nucleation on an inert substrate) and for a supersaturation
equal to unity (deposition on a native substrate) gives the value of the
crystallization overpotential,
It is equal to the difference of the
overpotential at point c and at point e in Fig. 3.1. If the current is switched
off at point e, the electrode potential will approach the reversible potential of
the deposited metal (point g); after switching on the current again at point g,
the overpotential returns to the same value as at point e, i.e. the deposition
overpotential,
meaning that a new phase is formed. On the contrary, if
current is switched off before point c, the electrode potential will approach
the initial stationary potential of the inert electrode, meaning that new phase
has not been formed.1
Using
t=25째C, n=l or 2 and Eq. 3.4 Klapka2 calculated the
dependencies of the nucleation overpotential on the
ratio for
2, 5 and 10. The calculated curves are shown in Fig. 3.2a. From these curves
the dependancies of the crystallization overpotentials on the
ratio, shown
in Fig. 3.2b, can be derived.

34

Chapter 3

The crystallization overpotential strongly decreases with increasing
ratio. As a results of this, it can be measured only in the case of a metal
deposition which is characterized by very high values of the exchange
current density2.
3.1.2

The nucleation exclusion zones

3.1.2.1 Basic definitions
Metal electrodeposition on inert electrodes begins with the formation of
separate growth centres until a continuous or disperse deposit is produced.
Once a nucleus of the depositing metal has been formed, the current flowing
causes a local deformation of the electric field in the vicinity of the growing
centre. As a result, an ohmic potential drop occurs along the nucleus-anode
direction. Considering the high dependence of the nucleation rate on the
overpotential, new nuclei would be expected to form only outside the spatial
region around the initial nucleus. In that region the potential difference
between the cathode and the electrolyte surpasses some critical value
Using simple mathematics, one obtains for the radius of the screening zone,
in an ohmic-controlled deposition:

where
is the critical overpotential for nucleation to occur,
is the ohmic
drop between the anode and cathode, f is a numerical factor and is the radius of the nucleus. The radius of the screening zone depends on the value of
both
and
At a constant
an increase in
leads to a decrease in the
radius of the screening zone, the same is true if decreases at constant
The radius of a nucleation exclusion zone can be calculated on the basis
of the following discussion, taking into account the charge transfer
overpotential also. If there is a half-spherical nucleus on a flat electrode, the
extent of the deviation in the shape of the equipotential surfaces which
occurs around it depends on the crystallization overpotential, current density,
resistivity of the solution and on the radius of the nucleus,
If the distance
from the flat part of the substrate surface to the equipotential surface which
corresponds to the critical nucleation overpotential,
is l, then this changes
around defect to the extent
as is presented in Fig. 3.3.
Therefore, in this region the current lines deviate from straight lines
towards the defect, thus causing an increase in the deposition rate, while in
the surrounding region nucleation does not occur, i.e., a nucleation exclusion
zone is formed. The voltage drop between the point from which the
deviation occurs and the nucleus surface. consists of the ohmic drop between

3. Surface Morphology of Metal Electrodeposits

35

these points and the charge transfer overpotential at the nucleus solution
interface. The nucleation overpotential includes both the crystallization and
charge transfer (deposition) overpotential:

Hence, at the moment when

become equal to l

where j is the current density along the current lines and is the electrolyte
resistivity. Hence, when the ohmic drop between the deviation point and
nucleus surface becomes equal to the crystallization overpotential, a new
nucleation becomes possible on inert substrate assuming in the both cases
the same charge transfer overpotential, and the same value of the current
density between the two symmetrical points on the anode and inert cathode
surface and between the same point on the anode and the point at the surface
of the earlier formed nucleus.
The radius of the nucleation exclusion zone,
corresponds to the
distance between the edge of a nucleus and the first current line which not
deviates (when
becomes equal to l). Accordingly, nucleation will occur
at distances from the edge of a nucleus equal or larger than
which can be
calculated as:

36

Chapter 3

If Eq. 3.7 is taken into account, one obtains:

According to Eq. 3.9, a new nucleation is possible in the vicinity of a
nucleus if
or
or
The analysis of the nucleation rate around a growing grain can also be
treated in a more rigorous way8. Regardless of this, the above model is
sufficient to explain the role of nucleation exclusion zones in the first stage
of electrocrystallization. This is because the nuclei formed are extremely
small and the spherical diffusion control around them can be established
after relatively large induction times9. During this induction time, the
nucleation exclusion zones are due to the ohmic drop in the vicinity of a
growing centre. At the same time, the nucleation process is practically
terminated, because it is very fast10. On the other hand, the rigorous
treatment of this problem is very complicated while the effect of the kinetics
parameters of the deposition process in the first stage of electrocrystallization can be qualitative explained in a simple way using the described
model, i.e. Klapkaâ&#x20AC;&#x2122;s concept of crystallization overpotential and the classical
nucleation theory.
During the cathodic process at low
the crystallisation overpotential is
considerably high; with increasing
however, it decreases rapidly2.
Hence, for
follows that
3.1.2.2 Physical simulation
The electrolytes used throughout the experiments were
in a
solution and
in a
solution to which ammonium hydroxide had been
added to dissolve the silver sulfate precipitate. The resistivity of the above
solutions are almost the same11.
It has been shown that silver deposition from a silver nitrate bath is under
pure diffusion control at all overpotentials, i.e.
For the ammonium
complex salt bath there is a well-defined region in which the deposition
process is under pure activation control

The silver grains obtained from the nitrate solution on a platinum
substrate are presented in Fig. 3.4.
In Fig. 3.5 the silver deposit obtained from the same electrolyte on the
substrate shown on Fig. 3.4 are presented.

3. Surface Morphology of Metal Electrodeposits

37

In Fig. 3.6 silver deposit from ammonium complex bath on the substrate
shown on Fig. 3.5 are presented. It can be seen from Fig. 3.5 that large
nucleation exclusion zones are formed around the initial grains during
deposition from the nitrate bath. They practically do not exist in the deposits
from the ammonium complex bath. In addition, new nucleation is seen on
the initial grain in Fig. 3.6.

In this way the effect of exchange current density of the deposition
process on the radius of the screening zone is clearly demonstrated.

38

3.1.3

Chapter 3

Nucleation rate and deposition overpotential

It has been established experimentally that the number of nuclei deposited
electrolytically onto an inert electrode increases linearly with time after an
induction period. After a sufficient length of time it reaches a saturated value
that is independent of time. The density of the saturation value increases with
the increasing applied overpotential and is strongly dependent on the
concentration of the electrolyte and the state of the electrode surface10.
Kaischew and Mutaftchiew13 explained the phenomenon of saturation on
the basis of energetic inhomogenity of the substrate surface. They assumed
that the active centres have different activity, or different critical overpotential
with respect to the formation of nuclei. Nuclei can be formed on those centres
whose critical overpotential is lower or equal to the overpotential externally
applied to the electrolytic cell. The higher the applied overpotential, the greater
the number of weaker active sites taking pan in the nucleation process and,
hence, the greater the saturation nucleus density. The formation and growth of
nuclei is necessarily followed by the formation and growth of nucleation
exclusion zones. After some time, the zones overlap to cover the substrate
surface exposed for nucleation, thus terminating the nucleation process10. This
is well illustrated in Fig. 3.7. It can be seen that the deposit obtained at low
current densities consist of a small number of nuclei but with increasing
overpotential or current density the number of growth sites increases and the
grain size of the deposit decreases.
The simultaneous action of both active centres and nucleation exclusive
zones must be taken into consideration when discussing the dependence of

3. Surface Morphology of Metal Electrodeposits

39

the number of nuclei on time. In the limiting case for active centres, when
screening zones are not formed, the saturation nucleus density is exactly
equal to the integral number of active centres. In the limiting case for
nucleation exclusive zones the saturation nucleus density is directly
proportional to the nucleation rate and inversely to the zone growth rate10. It
is obvious that the saturation nucleus density is larger in the first than in the
second case, because of the deactivation of active centres by overlapping
nucleation exclusive zones.

40

Chapter 3

The classical expression for the steady state nucleation rate, J, is given
:
by
1,14,15

where
and
are practically overpotential-independent constants.
Equation 3.10 is valid for a number of systems regardless of the value of the
exchange current density for the deposition process1,15. At one and the same
deposition current density, j, decreasing leads to an increasing nucleation
rate and decreasing nucleation exclusion zones radii. Hence, the limiting
case for nucleation exclusion zones can be expected when
and the
limiting case for active centres when
The saturation nucleus density, i.e., the exchange current density of the
deposition process, strongly effects the morphology of metal deposits. At
high exchange current densities, the radii of the screening zones are large
and the saturation nucleus density is low. This permits the formation of
large, well-defined crystal grains and granular growth of the deposit. At low
exchange current densities, the screening zones radii are low, or equal to
zero, the nucleation rate is large and a thin surface film can be easily formed.
The saturation nucleus density depends also on the deposition overpotential.
The nucleation law can be written16 as:

where

and
is the saturation nucleus surface density (nuclei
), being
dependent on the exchange current density of deposition process and the
deposition overpotential.
The overpotential and the current density in activation-controlled
deposition inside the Tafel region are related by:

Therefore, increasing
and decreasing
leads to an increase in the
deposition overpotential. According to Eq. 3.12, the value of A increases

3. Surface Morphology of Metal Electrodeposits

41

with increasing overpotential and decreases with decreasing exchange
current density. It follows from all available data that the former effect is
more pronounced resulting in deposits with a finer grain size with decreasing
value of the exchange current density.
Nucleation does not occur simultaneously over the entire cathode surface
but is a process extended in time so that crystals generated earlier may be
considerably larger in size than ones generated later. This causes periodicity
in the surface structure of polycrystalline electrolytic deposits, as well as
coarseness of the obtained thin metal film even when formed on a ideally
smooth substrate. Hence, the larger the nucleation rates, the more
homogeneous is the crystal grain size distribution, which leads to a smoother
deposit. Obviously, periodicity in the surface structure is a more complicated
problem, as was shown by Kovarskii et al17-19 but, for the purpose of this
analysis the above conclusion is sufficient. The purpose of this work is to
confirm the basic facts of the above theories and to show the effect of
exchange current density on the deposition process of thin metal film
formation on inert substrates.
3.1.4

Deposition from simple salt solutions

The polarisation curves for nickel, copper and cadmium deposition,
corresponding Tafel plots and the results of linear polarization experiments
are given in Ref. 12. The limiting diffusion currents in all cases are
practically the same, but the exchange current densities (given in Table 3.1)
are very different.

Electrodeposits of cadmium, copper and nickel are shown in Figs. 3.83.10, respectively. In the cadmium deposition, boulders were formed by the
independent growth of formed nuclei inside zones of zero nucleation. As a
result of the high value of the deposition overpotential is low and the
crystallisation overpotential is relatively large and so the screening zone,
according to Eq. 3.9, is relatively large. On the other hand, the nucleation
rate is low. This results in the deposits shown in Fig. 3.8.
In the case of copper, a surface film is practically formed by a smaller
quantity of electricity, as seen in Fig. 3.9, due to the lower exchange current

42

Chapter 3

density. The value of the deposition overpotential is larger than in the case of
cadmium and the crystallisation overpotential is lower, resulting in a
decrease in the zero nucleation zone radius, and hence a considerably larger
nucleation rate. A further decrease in the exchange current density value, as
in the case of Ni, leads to the situation shown in Fig. 3.10. A surface film is
formed, but it is porous, probably due to hydrogen co-deposition.

3. Surface Morphology of Metal Electrodeposits

43

Hence, a decrease in the value of the exchange current density of the
deposition process enhances thin surface metal film formation on inert
substrates due to an increase in the nucleation rate and a decrease in the
radius of the zero nucleation zones. As a result of this, a compact surface
metal film is formed with a smaller quantity of electrodeposited metal, and
its coarseness and porosity decrease with a decreasing exchange current
density. On the other hand, at sufficiently negative equilibrium potentials
and low hydrogen overpotential for an inert substrate, decreasing the
exchange current density of the deposition process can produce a porous
deposit due to hydrogen co-deposition.
3.1.5

Deposition from complex salt solutions

The silver deposits obtained from a nitrate bath at overpotentials
corresponding to an initial current densities of
and from the ammonium
complex salt bath at an overpotential corresponding to the current density of
are presented in Fig. 3. 1111. The solutions are the same as in section
3.1.2.2. It can be seen that the deposit obtained from the nitrate bath consists
of a small number of nuclei and that boulders are formed even at
which leads to the formation of a non-compact deposit.
On the other hand the deposit obtained from the ammonium complex salt
bath is microcrystalline.
The poor microthrowing power of deposits obtained from nitrate
solutions at smaller current densities can be explained in the following way.

44

Chapter 3

For

the deposition overpotential is given by:

according to Eq. 2.41, where:

and, for

and

from Eqs. 3.11 and 3.12.

Thus, at low current densities, poor microthrowing power is expected. In
the ammonium complex salt solution,
For
and
Eqs. 3.11,
3.12 and 2.37 are valid which means that N > 0. Hence, for deposition at low
current densities, decreasing
lead to increasing coverage of an inert
substrate for a given quantity of deposited metal.
3.1.6

Deposition in the presence of adsorbed additives

3.1.6.1
Inorganic compounds
In order to illustrate the effect, silver was deposited onto Ag and Pt
substrates from aqueous solutions containing
in 100 g

3. Surface Morphology of Metal Electrodeposits

45

Fig. 3.12a and
in
Fig 3.12b. Galvanostatic and potentiostatic deposition
conditions were applied in an open type cell20-22.

Silver deposits formed from a
containing electrolyte on to a Pt
substrate with galvanostatic current pulses are shown in Fig. 3.12b. At a
current density of
(i.e. under the optimal film deposition
conditions, as determined in Ref. 20), almost complete surface coverage was
achieved even with a charge quantity of
This is probably due to the possibility of further nucleation occurring
immediately next to the already existing nuclei, as a result of the smaller
values of the radii of the nucleation exclusion zones. Obviously, this is due
to the decrease of the exchange current density for the deposition process.
For comparison, in phosphate-free nitrate solution, a compact Ag film had
not been deposited even after
had been passed through the
cell, as can be seen from Fig. 3.12 a.20
3.1.6.2 Organic compounds
The electrolyte used in all experiments was a solution of
in
to which was added
poly(oxyethylene alkylphenol) (9.5 mol ethylene oxide)23.
The overpotential-log(current density) plot is given in Fig. 3.13. A
well-defined Tafel line characterised by
and

46

Chapter 3

was observed at higher potentials also. This phenomenon is explained
by the formation of a film of the organic additive which completely covers
the cathode at sufficiently negative potentials24,25. Tafel linearity was also
observed over a short overpotential range at low overpotentials. The values
of
and
obtained in this case are close to
the values expected for deposition from a pure solution26.

It has recently been shown27 that the optimum plating overpotential and
current density are determined by the upper limit of the validity of the
Tafel equation for the deposition process (see also section 3.2.1.3.1). In
this case, as can be seen in Fig. 3.13, the optimum deposition
overpotentials are about 530 mV and 40 mV in the presence and the
absence of adsorption of the aditive respectively. Cadmium deposits
thick obtained at 40 mV and 530 mV are shown in Fig. 3.14. It can be seen
that the deposits obtained at 40 mV have a large grain size, whereas those
obtained at 530 mV are fine grained, due to the larger overpotential.

3. Surface Morphology of Metal Electrodeposits

3.1.7

47

Conclusions

A surface metal film on an inert substrate is formed by the coalescence of
growing grains developed from corresponding nuclei, as is illustrated in Fig.
3.15, whereby the surface properties of the inert substrate are changed to
those of the electrodeposited metal.

48

Chapter 3

It is obvious that the larger nucleus density, the thinner is the thickness of
the metal film required to isolate the substrate from the solution. At the same
time a thinner surface film will be less coarse than a thicker one. This means
that a smoother and thinner surface film will be obtained at larger deposition
overpotentials and nucleation rates, i.e. by electrodeposition processes
characterized by high cathodic Tafel slopes and low exchange current
densities.
It is obvious that the discussion concerning the effect of the value of
exchange current density on the nucleation exclusion zone radius is not
connected to the mechanism of surface film formation but to the mechanism
of nucleation itself and the saturation nucleus number density. Papers
dealing with three-dimensional growth and related phenomena28 are mainly
concerned with the determination of the mechanism of the formation of a
surface film and are unimportant from the point of view of the estimation of
the deposit thickness required to isolate a substrate from an electrolyte
solution. For this purpose, the saturation nucleus density, or better to say the
distribution of the distances between nearest neighbours is much more
important. It is obvious that half of the largest distance between nearest
neighbours8,29 (as illustrated by Fig. 3.15) is the radius to which each grain
must grow to produce a nonporous thin metal film. It is clear that the
distribution of the distances between nearest neighbour crystallites is the
most important dependence in the treatment of the thin metal film formation
on an inert substrate. From the corresponding histograms, as shown by
Milchev et al8,29, it is possible to estimate the radius of the nucleation
exclusion zones, as well as the maximum distance between nearest
neighbour crystallites, which determines the thickness of a deposit required
to isolate the substrate from the electrolyte solution, as illustrated in Fig.
3.15. If the distance between the nearest neighbour crystallites is smaller
than the grain radius the deposit will overlap resulting in coarse deposit
growth initiation.
Apart from the nucleation density, the preferential orientation of the
nuclei is important in surface metal film formation. As the deposit becomes
free of the influence of the substrate structure on thickening, instead of the
formation of a randomly oriented grain structure, a preferred crystal
orientation can develop, which gives a definite texture to the cross section of
the deposit30. Texture can be expressed in terms of degree of orientation of
the grains constituting the deposit.
It is to note a the theoretical approach to the problems of deposit
orientation was successfully developed by Pangarov et al3-5. Using this

3. Surface Morphology of Metal Electrodeposits

49

theory, it was possible to determine the preferred orientation as a function of
overpotential from silver single crystals4 to nickel and iron thin films3,5.

3.2

THICK COMPACT METAL
ELECTRODEPOSITS

After formation of a thin surface metal film on an inert substrate further
deposition takes place in the same way as on a massive electrode of the
metal, the ions of which were reduced to form the electrodeposit. The final
thickness of the electrodeposited surface layer varies from the order of ten
micrometers in electroplating, to many times larger in electrowinning and
refining of metals. The surface of deposits obtained in electroplating must be
smooth and bright, in the other cases the surfaces of the deposit surface has
to be as smooth as possible. How this can be achieved will be revealed in
this section.
3.2.1

Coarse surfaces

3.2.1.1 Mathematical models
Any solid metal surface that represents a substrate for metal deposition
possesses a certain roughness. In addition, it may appear coarse or smooth,
and this is not necessarily related to the roughness. Fig. 3.16 shows cases of
surfaces with a) the same roughness and profoundly different coarseness and
b) vice versa.

50

Chapter 3

It is the level of coarseness which determines the appearance of metal
deposits, while even with considerable roughness, if below the visual level,
the surface may appear smooth.
It is convenient to define the surface coarseness as the difference in
thickness of the metal at the highest and lowest points above an arbitrary
reference plane facing the solution. In early models used to describe the
surface by periodic functions this is equal to twice the amplitude of the
function31,32.
Historically, it was first established that under certain conditions of
dissolution the surface coarseness tends to decrease33. Krichmar34 was the
first to point out that in some cases of deposition, under conditions
somewhat analogous to those in which the coarseness decreases, the opposite
effect occurs; i.e., in prolonged cathodic reduction, under conditions at
which, the process is close to being under complete diffusion control,
amplification of the surface coarseness occurs.
Taking a sinusoidal profile for the electrode surface:

Krichmar34 obtained the relationship:

for

where

and

In the above equations a is the wavelight of the sinusoidal profile, F is
the Faraday constant, H is the local elongation,
is the initial amplitude of
the sinusoidal profile,
is the amplitude of the sinusoidal profile at time
t, j is the current density, is the limiting diffusion current density, V is the
molar volume of metal, n is the number of electrons, Q is the quantity of

3. Surface Morphology of Metal Electrodeposits

51

electricity, t is the time, x is the co-ordinate normal to the plane of the
electrode and is the thickness of the diffusion layer.
Simpler mathematics were used in another, independently derived theory
of the same phenomenon put forward by
et al35, and Diggle et al36. A
somewhat simplified treatment will be given here.
Consider the model of the surface irregularity shown in Fig. 3.17. The
surface irregularity is buried deep in the diffusion layer, which is
characterized by a steady linear diffusion to the flat portion of the surface.
The current densities at the various parts of the surface are as follows.
a) At the flat part of the surface, the limiting diffusion current density is
that for steady-state linear diffusion, i.e.,

where D is the diffusion coefficient and
is the bulk concentration of the
depositing ions.
b) At the side of an irregularity, even when a possible lateral diffusion
flux supplying the depositing ions is neglected, the current density, at any
point of height must be larger than the current density, j, at the flat part of
surface.

52

Chapter 3

This is because the point is closer to the diffusion-layer boundary; i.e.,
the effective diffusion layer is thinner, and hence the diffusion flux and
resulting current density are larger. Obviously, this is valid if the protrusion
height does not affect the outer limit of the diffusion layer, i.e. if
The limiting diffusion current density,
is given as:

c) At the tip of an irregularity, there is an additional reason for an
increased current density. The lateral flux cannot be neglected, and the
situation can be approximated by assuming a spherical diffusion current
density,
given by:

where C* is the concentration of the diffusing species at a distance r from the
tip, assuming that around the tip a spherical diffusion layer having a
thickness equal to the radius of the protrusion tip is formed37,38. If deposition
to the rnacroelectrode is under full diffusion control, the distribution of the
concentration C inside the linear diffusion layer is given by36

where

Hence,

and from of Eqs. 2.29, 3.20, and 3.2239, it follows:

The general equation of the polarisation curve for a flat surface is given
by Eq. 2.28:

3. Surface Morphology of Metal Electrodeposits

53

The current densities and
to different points of the electrode surface
can then be obtained by substitution of in Eq. 2.28 by appropriate values
from Eqs. 3.19 and 3.23 for the side and the tip of the protrusion, around
which spherical diffusion layer is formed, respectively. Hence:

and

If a diffusion layer around the tip of a protrusion can not be formed (see
3.2.1,3.2):

The effective rate of growth of the side elevation is equal to the rate of
motion of the side elevation relative to the rate of motion of the flat
surface31. Hence:

Substitution of from Eq. 3.24 and j from Eq. 2.28 in Eq. 3.27 and
further rearrangement gives39:

54

if,

Chapter 3

and

or in the integral form:

where
is the initial height of the local side elevation just as in the
previous case (Eq. 3.16), Q is given by Eq. 3.17, and:

According to both mechanisms (Eqs. 3.16 and 3.29), an increase in the
surface coarseness can be expected with increasing quantity of deposited
metal for the same deposition current density, as well as with increasing
current density for the same quantity of electrodeposited metal.
In the same way, the propagation rate of the protrusion tip can be
obtained by substituting
from Eq. 3.25 and j from Eq. 2.28 into Eq. 3.27,
where and
are substituted by
and h, on further rearrangement the
following expression is obtained:

It should be noted that Eq. 3.31 is valid only if the radius of the
protrusion tip is sufficiently large to make the surface energy term
negligible37.
It is obvious from Eqs. 3.28 and 3.31 that

because
and
which means that the tip propagation
protrusion will be larger under spherical diffusion control.
3.2.1.2 Physical Simulation
To test the validity of the above equations,
and Popov40,41 carried
out experiments on diffusion-controlled metal electrodeposition on a welldefined, triangularly shaped surface profile, through a diffusion layer of

3. Surface Morphology of Metal Electrodeposits

55

well-defined thickness
A phonograph disk negative was used as the
substrate upon which a layer of an agar-containing copper sulfate-sulfuric
acid solution was placed and left to solidify, as illustrated in Fig. 3.18.

As current was passed and the layer was depleted of copper ions, an
increase in the height of the triangular ridges was observed. Metallographic
samples were made in wax and the cross sections of the deposit were
photographed under a microscope, Fig. 3.19 was thus obtained41.

In accordance with the discussion presented in Section 3.2.1.1, three parts
of the surface can be seen in Fig. 3.19: the flat part of the electrode and the
sides and the tips of irregularities, providing an excellent physical illustration
of the mathematical model.

56

Chapter 3

The effect of deposition time at a given current density, i.e., the effect of
the quantity of electrodeposited metal, on the protrusion height is obvious,
the larger the deposition time, the larger is the surface coarseness41.
3.2.1.3

Real Systems

3.2.1.3.1 Optimum current density for compact metal deposition
The Tafel plot for copper deposition is given in Fig. 3.20.

The surface coarseness for a fixed quantity of electrodeposited metal in
mixed activation窶電iffusion controlled deposition increases strongly with
increasing current density42,43.
Activation-controlled deposition of copper produces large grains with
relatively well-defined crystal shapes. This can be explained by the fact that
the values of the exchange current densities on different crystal planes are
quite different, whereas the reversible potential is approximately the same
for all planes44. This can lead to preferential growth of some crystal planes,
because the rate of deposition depends only on the orientation, which leads,
to the formation of a large-grained rough deposit. However, even at low
degrees of diffusion control, the formation of large, well-defined grains is
not to be expected, because of irregular growth caused by mass-transport
limitations. Hence, the current density which corresponds to the very
beginning of mixed control (a little larger than this at the end of the Tafel

3. Surface Morphology of Metal Electrodeposits

57

linearity) will be the optimum one for compact metal deposition, as follows
from Fig. 3.20.
All the above facts are illustrated in Fig. 3.2143.

3.2.1.3.2 Cauliflower like forms
It can be seen from Fig. 3.21c that the surface protrusions are globular
and cauliflower-like. If the initial electrode surface protrusions are ellipsoidal, they can be characterized by the base radius
and the height h as
shown in Fig. 3.22a.
The tip radius is then given by:

The initial electrode surface protrusion is characterized by
and
if
In this situation, a spherical diffusion layer cannot be formed
around the tip of the protrusion if
and linear diffusion control occurs,
leading to an increase in the height of the protrusion relative to the flat surface according to Eq. 3.29. When h increases, r decreases, and spherical
diffusion control can be operative around the whole surface of protrusion, if

58

Chapter 3

it is sufficiently far from the other ones, as illustrated by Fig. 3.22b. In this
situation, second-generation protrusions can grow inside the diffusion layer
of first-generation protrusions in the same way as first-generation
protrusions grow inside the diffusion layer of the macroelectrode, and so on.

A cauliflower deposit is formed under such conditions, as is shown in
Fig. 3.23. It can be seen from Fig. 3.23a that the distance between the
cauliflower grains is sufficiently large to permit the formation of spherical
diffusion zones around each of them. At the same time, second-generation
protrusions grow in all directions, as shown in Fig. 3.23b and c. This
confirms the assumption that the deposition takes place in a spherically
symmetric fashion.
To a first approximation, the rate of propagation can be taken to be
practically the same in all directions, meaning that the cauliflower-type
deposit formed by spherically symmetric growth inside the diffusion layer of
the macroelectrode will be hemispherical, as is illustrated in Fig. 3.23a-c.
This type of protrusion is much larger than that formed by linearly
symmetric growth inside the diffusion layer of the macroelectrode (Fig.
3.23a-c), as is predicted by Eq. 3.32.

3. Surface Morphology of Metal Electrodeposits

59

This is because a spherical diffusion layer cannot be formed around
closely packed protrusions, their diffusion fields overlap and they grow in
the diffusion layer of the macroelectrode.
3.2.1.3.3 Carrot like forms
It can also be seen from Fig. 3.23c,d and 3.24 that the growth of some
protrusions produces carrot-like forms, another typical form obtained in
copper deposition under mixed activation-diffusion control. This happens
under the condition r/h << 1, when spherical diffusion control takes place
only around the tip of the protrusion, as is illustrated in Figs. 3.17 and 3.24.
In this case Eq. 3.25, can be rewritten, if the surface energy effect is
neglected, in the form:

60

Chapter 3

meaning that deposition on the protrusion tip can be under pure activation
control at overpotentials lower than the critical one for the initiation of
dendritic growth (see section 3.3.2).

3. Surface Morphology of Metal Electrodeposits

61

This happens if the nuclei have a shape like that in Fig. 3.24a. The
assumption that the protrusion tip grows under activation control is
confirmed by the regular crystallographic shape of the tip45 just as in the case
of grains growing on the macroclectrode under activation control (see Fig.
3.21a).
The maximum growth rate at a given overpotential corresponds to
activation-controlled deposition. As a result, the propagation rate at the tip
will be many times larger than that in other directions, resulting in
protrusions like that in Fig. 3.24b. The final form of the carrot-like
protrusion is shown in Fig. 3.24c. It can be concluded from the parabolic
shape that such protrusions grow as moving paraboloids in accordance with
the Barton-Bockris theory37, the tip radius remaining constant because of the
surface energy effect. It can be concluded from Fig. 3.24d that thickening of
such a protrusion is under mixed activation-diffusion control because the
deposit is seen to be of the same quality as that on the surrounding
macroelectrode surface. It can be seen from the Fig. 3.24e that activation
control takes place only at the very tip of the protrusion.
Some of the new nuclei are precursors of carrot-like protrusions,
depending on their crystal orientation and position relative to the already
growing protrusion32. In this case, they are in the form of small hexagonal
pyramids, as shown in Fig. 3.24e. Based on their morphology and because
copper has a face-centred cubic crystal structure, it is reasonable to assume
that they are truncated by a high-Miller-index plane. According to Pangarov
et al3-5, the orientation of nuclei is related to the applied overvoltage. It is
reasonable to expect that the appearance of precursors of carrot protrusions
have its own overvoltage range. Obviously, such kind of protrusions can
produce short cuts and deposition of copper must be carried out at
overpotentials lower than this at which carrot like protrusion can be formed.
3.2.2

Smooth surfaces

3.2.2.1
Basic Facts
It is well known31 that in electrolytes containing specific substances as
additives, a phenomenon opposite to the ones described so far can occur, i.e.
a more rapid metal deposition at recessed points of the surface than at
elevated points. This causes levelling of the surface irregularities as is
illustrated in Fig. 3.25.
The fact that this phenomenon is only observed at microprofiles not
exceeding
in amplitude necessitated the introduction of the concept
of â&#x20AC;&#x153;microthrowing powerâ&#x20AC;? as a category different from ordinary throwing

62

Chapter 3

power31. The latter is used in technical literature to describe the quality of
electrolytes in plating on macroprofiles, at which a similar effect is never
observed. The difference between a microprofile and a macroprofile can be
seen from Fig 3.26.

3. Surface Morphology of Metal Electrodeposits

63

Detailed surveys of the literature on levelling are available in Refs. 31,
46, 48 and 49.
3.2.2.2
Model of leveling
All the experimental evidence points to the conclusion that leveling takes
place under conditions when the supply of the substance causing inhibition
of the electrode process is under diffusion control. It was already clear to
early investigators that the explanation should be sought in the local
variations in the supply of the leveling agent over the surface profile. Peaks
at the surface receive larger amounts of an additive than the recesses. This
results in an increase in inhibition and a decrease in the local current density
of deposition at the protrusion relative to less-exposed parts of the surface.
Thus, leveling is directly related to differences in the surface concentration
of the additive which leads to differences in the local current density of
deposition31.
The deposition current density of metal ions must be close to the end of
the Tafel linearity, i.e. it can be treated as the current density in activation
controlled deposition, being independent on the geometry of the system.
Hence, the current density at the tip of a protrusion, will be equal to the
current density on the flat surface in the absence of additive. It should be
noted that under such condition â&#x20AC;&#x153;geometric levelingâ&#x20AC;? occurs, but true
levelling requires the presence of an additive. If the additive is consumed at
the electrode by the reaction

the limiting diffusion current density of the additive,
to the tip of the
protrusion from Fig. 3.17, if spherical diffusion can be neglected, is given
by:

and that to the flat part of the electrode,

by:

where is the number of electrons in Eq. 3.35,
and
are the diffusion
coefficient and concentration of the additive, respectively.

64

Chapter 3

Assuming that the overall current density, j, at each point of the electrode
surface is equal, the effective current density,
of metal deposition at the
tip of a protrusion is given by:

and on the flat part of the surface by:

and following the same procedure as in the treatment of the increase of
coarseness (see section 3.2.1), one can write:

or in the integral form:

where:

where V (molar volume of metal) and n correspond to the electrodeposited
metal. This is the simplest mathematical model of the leveling process50.
Despite this, it elucidates the physical essence of the phenomenon under
consideration well.
Obviously, for this model to be operative, two conditions must be
satisfied: (a) the additive must be consumed in some manner at the electrode,
so that it must be continuously supplied in order to maintain a certain surface
concentration, and (b) the diffusion layer must not follow the microprofile
but must have a smoother outer boundary, so that variations in its thickness
arise, which cause variations of the diffusion flux of the additive.31
The first condition is fulfilled with all good levelling agents. Most of
them undergo sufficiently strong adsorption to remain long enough at the
metal surface to be surrounded by depositing atoms and be incorporated into
the deposit. It is the balance between the rate of incorporation and that of

3. Surface Morphology of Metal Electrodeposits

65

diffusion of the substance from bulk of solution which maintains a given
surface concentration of the additive. The larger the diffusion flux, the
higher is the steady-state surface concentration of the additive. Conversely,
higher rates of metal deposition cause a lowering of the latter.31
There is an optimal range of additive concentration and current density of
deposition at which the differences in inhibition of deposition between the
peaks and recesses, and hence the effect of levelling, are maximal. At too
low surface concentrations of the additive, i.e., low bulk concentration and
high current density of deposition, the process is practically uninhibited and
little difference in the local current density of deposition can arise. This
explains the decrease in the levelling effect with increasing current density.31
At somewhat higher bulk concentrations and lower current densities,
linearity exists between the bulk and the surface concentration. This is the
range of maximum difference in inhibition.31
However, at still higher concentrations, an adsorption/desorption equilibrium
tends to be approached leading to a Langmuir-type relationship. Eventually, in
spite of incorporation, saturation of the surface is reached and the surface
concentration is no longer sensitive to local changes in the diffusion flux of the
aditives. Hence, differences in inhibition vanish and leveling is lost.31
One should appreciate that some time is needed for the diffusion layer to
develop to the extent that it separates from the surface microprofile and provides
for local differences in the diffusion flux of the aditives. Hence, an induction
time should be expected before the leveling effect appears. This is demonstrated
by the observed sensitivity of the process to current interruptions48.
3.2.2.3 Quantitative Treatment
Krichmar51 made an attempt at a comprehensive quantitative consideration of the problem. He proposed that additives adsorbed to the surface of an
electrode are incorporated into the deposit at a rate proportional to the
surface coverage and current density.
For a sinusoidal profile of the electrode surface, Krichmar51 obtained an
exponential decrease of the amplitude,
with time:

where is a time constant given by:

where:

66

Chapter 3

K and
in the above equations are constants and other symbols have
meaning as in the Eqs. 3.18, 3.36 and 3.37.
An exponential decrease of the amplitude of the surface profile was
found experimentally by Krichmar and Pronskaya52.
Regardless, a model of the current distribution and numerical procedure
for the calculation of the change in shape of an electrode, for electrodeposition followed by diffusion-controlled reduction or incorporation of a
leveling agent, has been developed49,53-55, the approach of Krichmar51 seems
to be the most important one for understanding the leveling process.
3.2.3

Bright surfaces

3.2.3.1 Silver mirror
The surface of a silver mirror can be taken as the reference level for
bright surfaces56. It can be seen from Fig. 3.27, that a mirror surface consists
of parts parallel to the base and flat on the atomic level with low steps
heights between them, as is shown in Fig 3.28. Hence it can be expected that
bright metal surfaces must be similar to the surface of the mirror.

3. Surface Morphology of Metal Electrodeposits

67

3.2.3.2 Electropolished surfaces
The phenomenon of decreasing the surface coarseness of a metal upon
anodic dissolution under certain conditions is defined as electropolishing. In
cases when polishing occurs, the current-voltage curve was found to exhibit
a plateau characteristic for diffusion control of the dissolution process. Some
facts point to the complex nature of the phenomenon of electropolishing.
It was also found that systems undergoing electropolishing exhibit a
significant photoelectrochemical effect. This corresponds to the region of
limiting current densities and also to the maximum polishing effect.
This suggests to the existence of a photosensitive semiconducting film at
the surface and to a possible role of this film in the electropolishing process.
Subsequent measurements of the capacitance and resistance of the double
layer as functions of potential have shown that this film must be a very thin
and a well-conducting one.
In spite of all this, considerable evidence has accumulated justifying the
treatment of the electropolishing process as an essentially transportcontrolled phenomenon; this film could be related to the effect of
brightening31.
A quantitative model was suggested by Edwards57 and elaborated by
Wagner33. According to this model the metal ions produced are complexed
by a component of the electropolishing solution (e.g., phosphate ions or
water molecules). Hence, for the reaction to be completed, not only must
ions be formed, but also acceptor species have to diffuse to the surface from
the bulk of the solution in order to form a complex reaction product.
Diffusion of the acceptor from the bulk of the solution to the surface
determines the overall rate of reaction.
Hence, differences in acceptor fluxes of different points at the surface
arise. The slower diffusion of the acceptor to the recessed parts could cause
an increased concentration of free metal ions. This would have many

68

Chapter 3

possible consequences such as: increasing the cathodic partial current which
reduces the net dissolution current, producing changes in the reaction layer
such as the formation of an oxide film by hydrolysis, making room for an
additional phenomenon observed in electropolishing-brightening. This could
be related to the dissolution of facets and other crystallites. Brightening
seems to occur when the surface becomes covered by a protective film,
which controls the rate of dissolution and makes it a random process, the
energetic advantages of atoms at facets and dislocations being lost. Thus, it
could be concluded that the Edwards-Wagner model seems to provide a
reasonable basis for development of a comprehensive theory of the
electropolishing process.31
The reaction between a metal ion and a ligand is usually sufficiently fast
but it is the insufficient supply of the latter, which could cause the inhibition
of this step and make it the rate-determining step.
The anodic partial current is independent of the presence of the ligand.
However, if the ligand becomes scarce, the concentration of the free metal
ions increases and, as a result, the cathodic partial current is increased as
well. In an ideal case, when diffusion of free metal ions away from the
electrode is negligible, the difference between the two partial currents, i.e.,
the net dissolution current, must be proportional to the flux of the ligand.
This case was considered in detail by Wagner33 who assumed a molecular
diffusion mechanism of supply through a fixed hydrodynamic boundary
layer of thickness
much larger than wavelength and amplitude of the
surface profile.
For a sinusoidal profile of the electrode surface, Wagner33 gives:

where

is the initial surface amplitude, and is given by

a is the wavelength of the profile and
is the concentration of the acceptor.
Equation 3.47 shows that the electropolishing process is faster if the
thickness of the diffusion layer is smaller and if the bulk concentration of the
ligand is larger. Also, it is faster the smaller the wavelength of the surface
profile is. The latter reflects the radius of curvature of the elevations and
recesses and the result indicates that "microroughnees" will disappear more

3. Surface Morphology of Metal Electrodeposits

69

rapidly than "macroroughness", which is in accordance with experimental
experience (see also Fig. 3.30). The exponential time dependence of the
height of the elevations given by Eq. 3.43 is in agreement with the findings
of Krichmar and Pronskaya58. The electropolished metal surfaces are
characterized by a specular reflection of light.
Mirror reflection in a desired direction can only be obtained from a
suitably oriented flat metal surface, as one shown in Fig. 3.29.

In order to determine which structural features determine brightness, the
flat parts of the profile, which were parallel to the base, were examined
under different magnification. A part of the flat surface cross section was
investigated first at lower magnifications and then at increasingly higher
ones. Following this procedure Fig. 3.30 was obtained.
In the case of the mechanically polished surface, the amplitude of
roughness was several atomic diameters of Cu (Figs. 3.30a and 3.30b) whereas
the electrochemically polished surface exhibited smoothness on the atomic
level (Figs. 3.30c and 3.30d). The increase in the specular reflection of 20â&#x20AC;&#x201C;
25% is due to this fact. It is interesting to note (Fig. 3.31) that the structure of
the bright surface was not oriented. This is in accordance with the assumption
that the dissolution process under polishing conditions is a random process.

70

Chapter 3

Hence, it can be concluded that the condition for mirror brightness of a metal
surface is smoothness on the atomic level of a suitably oriented flat part of the
metal surface with low steps heights between, as in the case of a silver mirror.

3. Surface Morphology of Metal Electrodeposits

71

3.2.3.3 Electrodeposited surfaces
The same situation appears in the case of bright electrodeposits as is
illustrated in Fig. 3.32 and 3.33. The flat parts appear because the growth of
the deposit in the vertical direction is suppressed by the adsorption of the
additive60 and mirror-like surface is formed.

This happens if the organic additive adsorbs strongly on the top of the
copper crystallites and inhibits the formation of new growth centres.
Deposition then occurs dominantly at the edges of the crystallite, resulting in
lateral expansion of the layers, and the formation of smooth terraces on the
atomic level. The growth of a new layer starts from defect arising at the
interface between two adjanced crystallites and surfaces like those in Fig.
3.29 and 3.32 are formed.

72

Chapter 3

3.2.3.4 Conclusions
Finally, it can be concluded that increasing the overpotential at one and
the same current density leads to less coarse deposits with prolonged
deposition, because of lower grains formed during the formation of the
surface metal film. This conclusion is valid if
In the opposite case, a
rough deposit is often not formed because of disperse or granular deposit
formation. Hence, the first condition that must be satisfied in thick metal
film deposition is that the exchange current density must be considerably
lower than the limiting diffusion current density in the system under
consideration. The second conditions is that the deposition current density
must be a little larger than the one corresponding to the end of the Tafel
linearity. In this way, the formation of large and well-defined crystal grains
due to deposition under activation control will be prevented. At the same
time, the increase of the surface coarseness due to the deposition in mixed
activation-diffusion control will be minimal and the formation of carrot-like
protrusion will be avoided.
On the other hand smooth and bright deposits can be obtained in the
presence of organic additives only in the presence of additives which are
incorporated into the deposits or undergo electrochemical reaction, or, in the
presence of additives which are adsorbed on the flat part of the surfaces,
permitting the deposition at the steps. In fact, bright deposits are obtained by
the synergetic effect of the above two type of additives.

3.3

DISPERSE DEPOSITS

It was shown in the previous sections how compact electrodeposits are
obtained during electrodeposition at low degrees of diffusion control. In
condition close to complete diffusion control, disperse deposits are formed
by the mechanisms discussed below.
3.3.1

Spongy deposits

3.3.1.1 Mathematical Model
It follows from Eq. 2.28

that deposition in systems with low exchange current densities comes under
full diffusion control at sufficiently large overpotentials. On the other hand, if:

3. Surface Morphology of Metal Electrodeposits

73

deposition will be under complete diffusion control at all overpotentials if
some other kind of control does not take place (e.g., for silver deposition on
a well defined silver crystal grains at a silver electrode at low overpotentials
two-dimensional nucleation is the rate-determining step61).
At low overpotentials a small number of nuclei are formed, and they can
grow independently. The limiting diffusion current density to the growing
nucleus
is given by

or

where is the tip radius of the nucleus. Hence, if
the condition given
by Eq. 3.48 is not satisfied and deposition is under activation or mixed
control. Pure activation-controlled deposition is, thus, possible even at
on very small electrodes such as nuclei on an inert substrate.
An increase in leads to a decrease of
and, at sufficiently large
the deposition comes under mixed activation-diffusion control, i.e. when:

where is the radius of a growing nucleus where the process comes under
mixed control9,32.
Under mixed control of the deposition, amplification of the surface
irregularities on the growing nucleus occurs, leading to the formation of a
spherical agglomerate of filaments. Thereby a spongy deposit is formed. The
above reasoning is valid if spherical diffusion control can occur around
growing grains, as in the case of cauliflower deposit growth. Assuming that
around each grain with radius
growing under spherical diffusion control,
a diffusion layer of the same thickness is formed, then the initiation of
spongy growth is possible if the number of nuclei per square centimetre, N,
satisfies the condition

74

Chapter 3

On the basis of all the above facts, it can be concluded that the formation of a
spongy deposit on an inert substrate may be caused by mass-transport limitations when the nucleation rate is low. Hence, suitable conditions for the formation of spongy deposits arise at low overpotentials in systems where
The validity of the condition, given by Eq. 3.48 can be easily tested using
solutions of widely varying concentrations. The
values for
cadmium deposition from sulfate solutions are estimated as
for
(solution 1) and
for
(solution 2). The corresponding limiting diffusion currents can be assumed
to be
and
respectively. Hence,
and,
where the subscripts 1 and 2 correspond to solutions 1 and 2.
The cadmium deposit obtained from solution 1 is spongy as can be seen in
Fig. 3.34a, meaning that the conditions given by Eq. 3.48 and Eqs. 3.51 and
3.52 are all satisfied. In the case of solution 2, Eq. 3.48 is not satisfied and so
suitable conditions for the formation of a spongy deposit are not given.
Hence, the grains grow under pure activation control until a complete
surface film is formed, as illustrated in Fig. 3.34b, c9,32.

3.3.1.2
Physical Model
As was mentioned before, at a fixed value of the overpotential, the
growth of a spongy deposit is possible if:

The situation in which spongy deposits can start to grow can easily be
demonstrated62. Grains of the desired size and distribution can be grown at low
overpotentials under conditions of activation-controlled deposition. This

3. Surface Morphology of Metal Electrodeposits

75

corresponds to growth of grains when
The situation in which
can be
simulated by increasing the overpotential to a sufficiently high value to result in
diffusion control around the growing grains and the amplification of surface
irregularities. With increasing overpotential
decreases. This permits the
simulation of the initial stage of spongy growth, as is illustrated in Fig. 3.35.

The growth of protrusions in all directions is a good proof that the
deposition on the grain is under spherical diffusion control. At longer
deposition times, the protrusions branch and interweave, as is shown in Fig.
3.36, causing the macroelectrode to have a spongy appearance.

76

Chapter 3

3.3.1.3
Real Systems
Typical spongy electrodeposits are formed during zinc and cadmium
electrodeposition at low overpotentials9,32. Scanning electron microscopy
images of zinc deposited at an overpotental of 20 mV onto a copper
electrode from an alkaline zincate solution are shown in Fig. 3.37.
The increase in the number of nuclei formed with increasing deposition
time can be seen in Fig. 3.37a and b, and a spongy deposit is formed as can
be seen in Fig. 3.37b. The spongy growth takes place on a relatively small
number of nuclei, as is shown in Fig. 3.37b and c.

3. Surface Morphology of Metal Electrodeposits

77

The initiation of spongy growth at a fixed overpotential is possible if the
condition
(Eq. 3.51) is satisfied, which is the case after some time.
On the other hand, increasing the deposition time leads to the formation of a
larger number of nuclei, and so the condition given by Eq. 3.52 is not
satisfied over a large part of the electrode surface. Regardless of this, the
coverage of the electrode surface by spongy deposits increases with
increasing deposition time up to full coverage, as can be seen in Fig. 3.37d,
in the same way as was illustrated earlier by Fig. 3.36.

Spongy growth can start on the growing nucleus if the conditions given by
Eqs. 3.51 and 3.52 are both satisfied simultaneously.
In the first stage of deposition, the formation of nuclei having a regular
crystal shape can be expected because the deposition is activation-controlled.
After
is reached, the system comes under mixed control, producing
polycrystalline grains like those shown in Fig. 3.38a, just as in the case of
mixed control of copper deposition39, Fig. 3.21c. In this situation,
amplification of the surface irregularities on the growing grains occurs, and
spongy growth is initiated.
An ideal spongy nucleus obtained in a real system is shown in Fig. 3.38b
which illustrates the above discussion and physical simulation well63. The
agglomerate of filaments in Fig. 3.37b is obviously formed by further growth
of nuclei like that in Fig. 3.38b.
Hence, it can be concluded that at low overpotentials the initiation of
spongy growth is due to the amplification of surface protrusions directly
inside the spherical diffusion layer formed around each independently

78

Chapter 3

growing grain, as in the case of the formation of cauliflower deposits. The
growth of protrusions in all directions is good proof that the initial stage of
deposition on the grain is under spherical diffusion control, while further
growth takes place in the diffusion layer of the macroelectrode. In less ideal
situations, non-ideal spongy nuclei are formed, which, however, after further
deposition result in a macroelectrode with the same appearance.
It should be noted that some other possible mechanisms of spongy
deposit formation have been considered in a qualitative way, as reviewed in
Refs. 64 and 65, but the mechanism presented above seems to be the most
probable65. However, the mechanism of formation of a spongy deposit over
an initial coating, which is not seen in the case of cadmium but occurs in
zinc deposition 9,63 requires clarification. For instance, the mechanism of
spongy growth initiation in this case has not been elucidated.
3.3.2

Dendritic deposits

3.3.2.1
Mathematical Model
Two phenomena seem to distinguish dendritic from carrot-like
growth31,32:
1. a certain well-defined critical overpotential value appears to exist
below which dendrites do not grow,
2. dendrites exhibit a highly ordered structure and grow and branch in
well-defined directions. According to Wranglen66, a dendrite is a skeleton of
a monocrystal and consists of a stalk and branches, thereby resembling a
tree.
It is known that dendritic growth occurs selectively at three types of
growth sites31:
1. dendritic growth occurs at screw dislocations. Sword-like dendrites
with pyramidal tips are formed by this process31,36.
2. many investigations of the crystallographic properties of dendrites
have reported the existence of twin structures67-69. In the twinning process, a
so-called indestructible re-entrant groove is formed. Repeated onedimensional nucleation in the groove is sufficient to provide for growth
extending in the direction defined by the bisector of the angle between the
twin plants31,32.
3. it is a particular feature of a hexagonal close-packed lattice that growth
along a high-index axis does not lead to the formation of low index planes.
Grooves containing planes are perpetuated, and so is the chance for extended
growth by the one-dimensional nucleation mechanism70.
In all the above cases, the adatoms are incorporated into the lattice by
repeated one-dimensional nucleation. On the other hand deposition to the tip
of screw dislocations can be theoretically considered as deposition to a point;
in the other two cases, the deposition is to a line.

3. Surface Morphology of Metal Electrodeposits

79

From the electrochemical point of view, a dendrite can be defined as an
electrode surface protrusion that grows under activation control, while
deposition to the macroelectrode is predominantly under diffusion
control31,32,36. The polarisation curve for a flat macroelectrode is given by Eq.
2.28

and that for the tip of a protrusion growing inside the macroelectrode
diffusion layer is given by Eq. 3.25

It follows from Eq. 2.28 that
if
and
On the other
hand, if h >> r and
activation-controlled deposition to the tip of
the protrusion takes place, and Eq. 3.25 can be rewritten in the form:

where
is the corrected value of the exchange current density. This is so
because if r/h << 1, Eq. 3.22

can be rewritten in the form

where
is the concentration at the tip of a protrusion growing under
activation control inside the diffusion layer of the macroelectrode if the
deposition to it is under full diffusion control36. A similar situation can arise
if cylindrical diffusion around the tip of a growing protrusion occurrs71.
The exchange current density
at the tip of such a protrusion is given
by72:

80

Chapter 3

or, taking into account Eq. 3.54,

where is a function of the symmetry factor and is the exchange current
density corresponding to the bulk concentration of the depositing ion.
Assuming for the sake of simplicity that is approximately 1, Eq. 3.53 can
then be rewritten in the form:

Dendrites grow faster than the flat electrode surface, and the condition
for the initiation of dendritic growth is:

where
corresponds to
the critical overpotential at which initiation of
dendritic growth from the tip of a growing protrusion, whose height is h, is
instantaneously possible after the steady-state concentration distribution
inside the diffusion layer of the macroelectrode has been reached. Taking
into account the relationship:

Eq. 3.58 can be rewritten in the form:

The minimum overpotential at which dendritic growth can be initiated,
corresponds to
and Eq. 3.59 becomes:

3. Surface Morphology of Metal Electrodeposits

A different situation arises if
and
The current density to the tip of the protrusion is then given by:

for
given by

81

but

while the diffusion current density to the macroelectrode is

Assuming that
can be used instead of
and following the same
reasoning as in the derivation of Eqs. 3.58 and 3.60, one obtains:

It should be noted that all the above derivations are valid, if the
protrusion tip radius is sufficiently large to make the effect of the surface
energy term negligible37, and if Eqn. 2.32 is also valid.
It follows from Eq. 3.63 that for systems with
dendritic growth is
possible at all overpotentials. Experimentally, some critical overpotential of
dendritic growth initiation exists in all cases, being of the order of 10
37,73,74. Assuming that under complete diffusion and surface energy
mV
control
the current density to the macroelectrode is given by37:

and, assuming that Eq. 3.54 is valid, the current density on the tip of a
dendrite growing inside the diffusion layer of a macroelectrode is given by:

then, it is possible to derive the relationships:

82

Chapter 3

and

using the same procedure as in the derivation of Eqs. 3.59 and 3.6075.
3.3.2.2 Physical Simulation
The cross sections of the copper deposits obtained in a model system,
described earlier (Fig. 3.18), are shown in Fig. 3.39.

Deposits at 300 mV are compact; at 600 mV they are dendritic. This
means that dendrites are formed at overpotentials larger than a certain
critical value, as required by Eq. 3.60, because both overpotentials
correspond to the plateau of the limiting diffusion current. It is seen that the
current density to the tips of dendrites depends on the
ratio (see Eq.
3.57), so that larger dendrites are produced at more elevated points of the
electrode surface. This is because the effective height of the dendrite
precursor in the modelled diffusion layer is equal to the sum of the height of
the precursor and the height of the point at which nucleation took place
relative to the flat part of the electrode surface. In the same way, for nuclei
formed on the tip of a protrusion (Fig. 3.39b),
(see Eq. 3.59) is lower
than for those formed on the flat surface, and a dendrite is formed at the tip
of the protrusion while at the same overpotential dendrites are not formed on
the flat part of the electrode.

3. Surface Morphology of Metal Electrodeposits

83

The validity of Eq. 3.60 can be qualitatively tested by using the same
solutions (1 and 2) as where used for the examination of spongy deposit
formation. In this way, different
ratios for the same deposition process
can be obtained, while the surface energy and the crystallographic properties
of the metal are kept the same. As expected, because of the lower
ratio,
dendrites appear at lower overpotentials from the more dilute solution than
from the more concentrated one. This is illustrated in Fig 3.40.

3.3.2.3
Real Systems
There is an induction period before the initiation of dendritic
growth31,32,37. During this induction period, dendrite precursors are formed
and become sufficiently high to satisfy Eq. 3.59 at a given overpotential, as
illustrated in Fig. 3.41. The crosslike grains seen in Fig. 3.4la and b further
develop into dendrite precursors (Fig. 3.41a, c).
The propagation of this structure by branching (Fig. 3.41d) produces
dendrites as shown in Fig. 3.41e.
The initiation of dendritic growth is followed by a change in the slope of
the current density-time curves31,32, indicating a change in the growth
mechanism of the deposit.
The slopes of these dependences are similar to each other and
independent of the deposition overpotential during the non-dendritic
amplification of the surface-coarsening.
The change of the slope of the current-time dependences due to the
dendritic growth initiation will be treated here in somewhat simplified way.
The limiting diffusion current density to the tip of a surface protrusion,
is given by75:

84

if the spherical flux around the tip can be neglected, and:

Chapter 3

3. Surface Morphology of Metal Electrodeposits

85

to the flat part of the electrode.
Differentiation of Eq. 3.68 gives:

and as in Eqn. 3.27

taking into account Eqs. 3.68 and 2.29 if
Substitution of dh/dt from Eqn. 3.70 in Eqn. 3.69 produces

being independent on overpotential.
After initiation of dendritic growth, the slopes become dependent on the
overpotential. A dendrite is a surface protrusion growing under mixed or
activation control, while deposition to the flat part of the electrode surface is
under complete diffusion control. The overpotential and current density
on the tip of a dendrite are related by:

Differentiation of Eq. 3.57 produces:

and as in the derivation of Eqn. 3.71

and

86

Chapter 3

Hence, the maximum overpotential at which the slope of the apparent
current density-time dependence remains constant and equal to that in
nondendritic amplification of the surface-roughness corresponds to
The
minimum overpotential at which this slope cannot be recorded corresponds
to
In this way and
can be estimated. It is known that the j-t dependence
are different from case to case owing to different mechanisms of dendritic
growth initiation and dendritic growth31,32. As a result of this, the analytical
approach to the determination of and
must be specific for each system
under consideration; the procedure for one particular case is as follows.
Typical log (current)-time dependences obtained for copper deposition
from
in
at overpotentials belonging
to the limiting diffusion current plateau are shown in Fig. 3.42. According to
the above discussion, it is clear that the intersection points of the two linear
dependencies determines the induction time of dendritic growth initiation75.

3. Surface Morphology of Metal Electrodeposits

87

The induction times for dendritic growth initiation extracted from the
graphs in Fig. 3.42 can be presented as a function of overpotential, and the
critical overpotential for instantaneous dendritic growth can be obtained by
extrapolation to zero induction time.
The critical overpotential of dendritic growth initiation can be determined
by plotting the logarithm of the slopes of the straight lines from Fig. 3.42 as
a function of overpotential and the intersection point of the two straight lines
determines
A similar procedure was followed for the deposition of
cadmium from
in
The cross sections of the copper and cadmium deposits obtained at
and
are shown in Figs. 3.43a-c and 3.44a-c , respectively.
It can be seen that there is no dendrite formation when
both compact
and dendritic deposits are formed when
and only dendritic metal
is deposited when
This is in perfect agreement with findings of
Calusaru77 for the morphology of deposits of the same metals deposited at
overpotentials corresponding to full diffusion control.
The and
of 260 mV and 660 mV for copper deposition (lower value) and 27 mV and 110 mV for cadmium deposition (larger value), are
successfully determined using the above given procedure, being in perfect agreement with experimental findings as can bee seen from Figs 3.43 and 3.44.75

88

Chapter 3

It is known78 that, apart from decreasing the concentration of the depositing
ion, the formation of a dendritic deposit can also be enhanced by increasing
the concentration of the supporting electrolyte, increasing the viscosity of the
solution, decreasing the temperature, and decreasing the velocity of motion of
the solution. Practically, all the above facts can be explained by Eqs. 3.60 and
3.63, assuming that a decrease in
means enhanced dendrite formation
because of the lower electrical work required to produce the dendrites. The
possibility of obtaining dendrites of Pb73 and Sn74 from aqueous solutions at
lower overpotentials than required for the formation of dendrites of Ag from
aqueous solutions can also be explained by Eq. 3.67 owing to the much lower
melting points of these metals, i.e., their lower surface energy at room
temperature. Dendrites of silver can be obtained from molten salts at
overpotentials of a few millivolts37, as in the case of Pb and Sn deposition
from aqueous solutions73,74, because the difference between the melting point
of silver and the working temperature for deposition from molten salts is not
very different from the difference between the melting point of lead or tin and
room temperature. On the other hand, dendrites grow from screw dislocation
and nuclei of higher indices or twinned ones only31,32. The probability of
formation of such nuclei increases with increasing overpotential79 and
can
also be defined as the overpotential at which they are formed. Regardless of
this, Eqs. 3.60, 3.63 and 3.67 illustrate well the effect of different parameters
on the initiation of dendritic growth.

3. Surface Morphology of Metal Electrodeposits

89

It is obvious that the electrochemical conditions, as well as the crystallographic ones, under which dendritic deposits are formed can be precisely
determined. One problem that still seems to remain unresolved is the question of
what causes the dendrite precursors to appear at regularly spaced locations along
the dendrite stem. Further investigations in this direction are necessary.31
3.3.3

Powdered deposits

A metal powder represents a dendritic deposit which can spontaneously
fall or can be removed from the electrode by tapping or in a similar way.
All metals, which can be electrodeposited, exhibit a tendency to appear in
the form of powders at current densities larger than a certain critical value
This value is equal to the limiting diffusion current density in galvanostatic
deposition, as was shown by Ibl80 . Simultaneously it was observed that the
product of the employed current density and the square root of the time of
powder formation is a constant quantity31. Such dependencies are characteristic for processes controlled by diffusion and the time of powder formation
coincides with the transition time. The time for powder formation at current
densities equal to or larger than can be observed visually as the appearance
of the electrode is seen to turn suddenly from lustrous to black.
It is known that increasing the overpotential leads to the formation of a
more dispersed deposit characterised by decreased particle size, even at the
same initial current density (and real current density in potentiostatic
deposition) because increasing the overpotential means the increasing the
electrical work, thus a powder with larger specific surface area is produced.
This is illustrated in Fig. 3.45, where copper particles obtained at
different overpotentials are presented81.
In the same way, the differences in the grain size of the powder particles of
different metals can be explained assuming that their surface energies are
similar. It can be seen that an increase in
and the
ratio leads to an
increase in and, hence a decrease in the grain size of powder particles can be
expected as is illustrated in Fig. 3.45a and Fig. 3.46a. In the same way, the
different grain sizes of the same metal powder particles but obtained from
different electrolytes can be explained, as is demonstrated in Fig. 3.46. It was
shown earlier11, that deposition of Ag from
in
is characterised by
and the deposition of silver from
in
is characterized by
as in
the case of copper. It is also noteworthy that in soft metal (low melting points)
powder deposition agglomerates are formed due to the plasticity of the
growing dendrites, as can be seen in Fig. 3.47.
The effect of deposition conditions on the grain size of powder particles
can not be discussed using Eqs. 3.59 and 3.60 alone. Despite this, in all cases
increasing the overpotential leads to the formation of smaller particles and to a
narrower particleâ&#x20AC;&#x201C;size distribution curve. It was shown that changing concen-

90

Chapter 3

tration of the electrolyte83 or the stirring rate84 do not affect appreciably the
and
values. Also, increasing the temperature leads to an increase in both
and
as dues increasing the concentration, and a significant effect on the
value of and
is not to be expected. In these cases, however, deposition
at a similar overpotential means deposition at very different deposition current
densities. Consequently, an increase in the particles grain size is to be
expected, for the same deposition time, with increasing concentration, temperature, and decreasing concentration of the supporting electrolyte, as is illu-

3. Surface Morphology of Metal Electrodeposits

91

strated in Fig. 3.48. Stirring, according to
et al84 has the same effect
as increasing the concentration, as well as increasing the deposition time.

92

Chapter 3

A large difference in the quality of a powdered deposit can arise in
prolonged deposition due to different cathode materials85 because of the
different properties of the solid-solution interface, as well as in galvanostatic
and potentiostatic cases.
The use of either the potentiostatic or galvanostatic methods result no
substantial difference in the deposition of metal powders on different
cathode materials. The only real difference lies in the morphology of the
powder particles obtained by potentiostatic and galvanostatic deposition86,
i.e. that the particles obtained by galvanostatic deposition are less dendritic
than those obtained by potentiostatic deposition, because the overpotential at
the end of the deposition is less negative than in this case.
3.3.4

Granular deposits

Granular deposits consisting of independently growing grains with a
highly developed surface area are obtained by deposition of metals in
processes characterised by large values of the exchange current density87-89.
In some cases they consist of grains growing independent of each other until
a compact film9,90 or spongy deposit on it is formed9.
On the basis of the facts concerning the formation of a surface film and
the initiation of dendritic growth, the mechanism of the formation of a
granular deposit can easily be proposed.
It is shown in Fig. 3.49 that various crystallographic forms, some of them
ideal, are obtained during silver deposition at low overpotentials from pure
silver nitrate solution, due to the independent grain growth inside the zones
of zero nucleation, probably if 2D nucleation is the rate determining step61.
Dendritic growth starts at higher overpotentials. It can be seen from Fig.
3.49b and c that dendritic growth initiation is mainly related to the
appearance of twinned forms in which indestructible re-entrant groove is
formed, being the precursors of dendrite. Hence, a granular deposit will be
formed at overpotentials lower than those for the formation of dendrite
precursors and dendritic growth initiation. The critical overpotential of
dendritic growth initiation increases with increasing ion concentration, being
120 mV in
on a silver substrate20. The grains deposited
from this solution at 100 mV are different in size, and less ideal than in the
previous case, as can be seen from Fig. 3.50a. From Fig. 3.50b and c it can
be seen that higher protrusions block the further growth of the lower ones
and the formation of a surface film becomes impossible. The granulae will
grow towards the bulk solution because lateral flux can be neglected and
granular deposits are formed22. This is due to the formation of depletion
zones around the growing grains91.

3. Surface Morphology of Metal Electrodeposits

3.3.5

93

Whisker deposits

This form of crystal growth differs from that of dendrites in that (a) it
tends to have a still larger ratio between the longitudinal and the lateral
dimensions with an almost perfect preservation of the latter during the

94

Chapter 3

growth, and (b) it exhibits no tendency to sidebranching as can be seen from
Fig 3.51. Impurities or additives in the electrolyte seem to be a prerequisite
for its appearance31,92.

Gorbunova et al94,95 grew silver whiskers from fairly concentrated silver
nitrate solutions
containing oleic acid, gelatine, albumin,
and heptyl, octyl, and nonyl alcohols.
A few more phenomena should be noted: a) while growing exclusively in
one direction only, whiskers dissolve anodically at a practically uniform rate
from all sides92 and at an overpotential much smaller than that needed for
growth; b) a higher overpotential is needed temporarily for the initiation of
growth (or continuation after interruption) than for growth at a steady rate; c)
if the growth is interrupted for a longer period of time, then it may continue
at the tip, but usually assuming a new direction, or else it may be completely
prevented and a new whisker started elsewhere. The minimum time required
for complete cessation of further growth was found to depend on the
concentration of the additive; (d) if a constant rate of growth is maintained,
by a constant current flow through the cell to the individual whisker tip,
fluctuations of overpotential are observed.
Finally, it should be noted that whiskers differ from other crystals of the
same metal in two respects at least: they have an increased electrical
resistivity (2-3 times that of crystals deposited in the absence of additives)
and an increased tensile strength
compared to a few
hundred
observed in large, pure silver single crystals).31
A model of the growth mechanism was developed by Price et al92 which
gives a good account of most of the phenomena observed. The basic

3. Surface Morphology of Metal Electrodeposits

95

assumption of the model is that molecules of impurities or additives are
strongly adsorbed at all but one crystal plane and at such a concentration as
to completely block the deposition and extension of the lattice. On the one
plane, however, the process of adsorption is competitive with that of metal
deposition whereby the adsorbed molecules are buried and, at a steady state,
a sufficiently low surface coverage of foreign molecules is maintained for
growth to be possible. The latter is assumed to occur by continuous
nucleation and movement of steps over the close-packed surface. Indeed, the
appearance of some whiskers, as for example, the one shown in Fig. 3.51,
suggests repeated one-dimensional nucleation of the type shown in Fig. 2.8,
and the extension of the step in two direction to the edge of the crystal.
3.3.6

Conclusions

It is obvious from the above discussions that the formation of disperse
deposits is required only in electrodeposition of metal powders. In this
situation, the deposition must be under complete diffusion control. Other
types of disperse deposits are undesired and their appearance can easily be
avoided by decreasing the exchange current density of the deposition
processes (by complexing depositing ions or by appropriate organic
additives) and maintaining the deposition overpotential below the values of
the critical overpotential for dendritic growth initiation. It seems that in the
case of whiskers nothing can be said in advance about their appearance.

It is a known fact that different morphologies of electrodeposited metal
can appear at the different positions of the electrode surface. This means that
the local current density during electrodeposition of a metal varies from
point to point on an electrode surface. This is due to the following factors1,2:
the geometry of the system;
the conductivity of the solution and electrodes;
the activation overpotential;
the diffusion overpotential;
the hydrodynamics of the system.
Although all factors effect the current distribution simultaneously, there
are three main types of current distribution on a macroprofile.
If the influence of overpotential is negligible, primary distribution is
determined by the geometry of the system and the conductivity of the
solution.
In the case of the secondary and tertiary distribution the activation
overpotential and both the activation and diffusion overpotentials have to be
taken into consideration, also.
Even for a simple electrode configuration, the calculation of the current
distribution is a complex problem and the difficulties further increase with
increasing complexity of the geometry, especially if the limiting diffusion
current varies over the electrode due to the different geometric and
hydrodynamic conditions. Because of this, analytical solutions can be found
only for some cases (Wagner3, Newman4,5), while in other cases numerical
solutions are available1. If the complete calculation can not be performed, it
is possible to estimate certain trends, using a dimensionless group called the
Wagner number,
given by
101

102

Chapter 4

where
is the slope of the cathodic activation overpotential-current
density dependence, Îş is the conductivity of solution and l is a characteristic
length.
Before
the parameter

was used, according to Kasper6, Hoar and Agar7.
The Wagner number represents the ratio of the polarisation resistance to
the solution resistance. The larger it is, the more even is the current
distribution in spite of non-uniform geometry. In general, the current
distribution is more uniform if1:
the smaller characteristic length of the system is,
the larger the conductivity of the solution is, and
the larger the slope of the activation overpotential-current density
curve is.
Obviously, the Wagner number can be used only to compare the current
distribution in the cell with non-uniform geometry, which contains different
electrolytes.
The same situation appears if the ability of an electrolyte to uniformly
distribute the current density is experimentally determined using the method
of Haring and Blum8.
The current distribution on a macroprofile is very important in technical
metal electrodeposition. In electroplating, the current distribution determines
the local variations in the thickness of the coating. In electrowinning and
electrorefining of metals, a non-homogenous current distribution can cause a
short circuit with the counter electrode, and the corner weakness effect in
electroforming. This is very important in the three-dimensional electrodes, as
well as in some storage batteries. In all the cited cases a uniform current
density distribution over the macroprofile is required.
The aim of this chapter is to present the procedure, based on simple
equations of electrode kinetics, by which the condition in which a desired
current density distribution can be obtained, or an undesired one avoided,
under the assumption that the limiting diffusion current density does not vary
over the whole electrode surface, including the edges of flat and the tips of
wire electrodes.

4. The Current Distribution in Electrochemical Cells

4.1

103

TWO EQUAL PLANE PARALLEL ELECTRODES
ARRANGEMENT

The cell with two equal plane parallel electrodes represents the
elementary cell of electrode arrangement in electrochemical refining and
winning processes.
It is a well-known fact that in a cell with parallel electrodes (if the
electrode edges do not touch the side walls of the cell), the current density is
higher at the edges than at the centre of the electrode1,9. This is because the
current flow passes partially around the rectangular space between the
electrodes. The increased current density at the edges of the electrodes can
be easily noticed by observing the quality of the metal electrodeposit at the
cathode. In some cases the deposit in the central part of the cathode may be
compact and flat whereas the occurrence of dendrites is observed at the
edges. The appearance of dendrites at the edges of the cathodes in such
situations is the most important problem of the current density distribution,
because the growing dendrites could cause short circuits followed by a
decrease in the current efficiency, or even damage the power supply.
The aim of this section is to show in which way dendritic growth at the
cathode edges can be avoided in electrowinning and refining processes.
4.1.1

Ohmic resistance of the cell

The current density distribution in a rectangular electrolytic cell in which
parallel electrodes cover only part of the wall is illustrated in Fig. 4.1.

104

Chapter 4

The linear approximation of the current distribution in the cell with plane
parallel electrodes shown in Fig. 4.1 is presented schematically in Fig. 4.2.

The analysis performed here for the current distribution between the
electrode edges and the cell side walls is obviously valid also for the
situation in which there is the distance between the upper edges of the
electrodes and free surface of ssolution and lower edges to the bottom of the
cell. In the case under consideration these two distances are zero.
The resistance dR of a section of the electrolyte of thickness dc is given
by:

where B is the height of the electrode and
electrolyte. From the linear approximation:

is the specific resistance of the

is obtained. The parameters d and c are indicated in Fig. 4.2.
The resistance of the whole electrolyte is then given by10:

and for

by:

4. The Current Distribution in Electrochemical Cells

105

where
corresponds to the resistance of a system with a homogeneous
current density distribution (the side walls touch the edges of the electrodes).
For
L can be related to A by a linear coefficient k as follows:

which transforms Eq. 4.3 to:

and

taking into account Eq. 4.4, where
represents the interelectrode distance
in a cell with L=0, the resistance of which is equal to the resistance of a cell
in which the interelectrode distance is l and L > 0
The cell for the determination of the equivalent resistance is shown in
Fig. 4.3.

The side screens enabled the distance L between the edges of the
electrodes and the side walls to be varied for given values A and 2C. The
resistance of the system for various adjusted values of L was measured by
the bridge method using platinum electrodes in a
KC1
solution. The electrodes were 2 cm long and 1 cm wide. The interelectrode
distance was 2 cm. Hence, in this case A=C. The back sides of the electrodes
were insulated. The upper edges of the electrode touch the free surface of the
solution and the lower edge the bottom of the cell.

106

Chapter 4

The dependence of the total resistance of a system with plane parallel
electrodes on the distance between the electrode edges and the cell side walls
is shown in Fig. 4.4.

The open circles represent the experimental values, arid the curve is
obtained using Eq. 4.6. The good agreement between the experimental
results and the values predicted by Eq. 4.6. extends to
It can be
concluded that for this system Eq. 4.6. is valid for k < 1. This means that the
maximum penetration of the current lines occurs when L = C = A in this
case, and that the maximum length of the current line, lâ&#x20AC;&#x2122; is
4.1.2

The very edge ohmic resistance

This consideration of the very edge current density can be elaborated
mathematically in the following way11. Assuming total ohmic control, the
voltage drop in the solution between the electrodes inside the homogenous
field is given by:

and outside of the homogeneous field by:

4. The Current Distribution in Electrochemical Cells

107

where U is the cell voltage, E is the equilibrium potential difference, is the
specific resistivity of the electrolute, l the interelectrode distance, j is the
current density, is the length of the i-th current line and is the current
density corresponding to the i-th current line, as can be seen from Fig. 4.5.

The difference in the current lines outside of the homogeneous field is
given by:

or in the differential form

When Eq. 4.11 is integrated from the inter-electrode distance l to the
maximum length of the current line, l', j', the maximum contribution to the
edge current density due to current line propagation between the electrode
edges and the side walls of the cell, is obtained:

108

Chapter 4

Taking into accounts Eq. 4.8 one obtains

The edge current density

can be written as

The maximum value of j' is obtained from Eq. 4.13 as:

Combining Eq. 4.15 and 4.14, the maximum edge current density can be
given as12:

for
as follows from Figs. 4.2 and 4.4.
This means that the very edge resistance is lower than in the homogenous
field and that the minimum effective interelectrode distance,
between
the edges of the anode and cathode will be:

because of

4.1.3

The edge effect

In a cell with parallel plate electrodes, if the electrode edges do not touch
the cell side walls, the potential difference between two points in the
homogenous field symmetrically positioned on the electrodes is given by:

Analogously, the cell voltage at the edges can be expressed as:

4. The Current Distribution in Electrochemical Cells

109

where
and
are, respectively, the anodic and cathodic overpotentials
corresponding to the homogenous field, and
and
are, respectively,
the anodic and cathodic overpotentials corresponding to the edges.
Elimination of U from Eqn. 4.19 and 4.20 gives

In this case
because the increasing of the current density
leads to the increasing of the cathodic and anodic overpotentials also.
In this way, a part of the ohmic potential drop in a homogenous field
transforms into electrochemical overpotential for points at the plane
electrode edges, or in a similar position, meaning the edge current density is
larger than in the homogenous field. In this way it is possible to explain the
change in the quality of the metal deposit near the edge and at the very edge
of an electrode. It should be noted, however, that according to the proposed
model the entire edge current is located at the very edge of the electrode. In
other words, a homogeneous electric field and, consequently, a uniform
current distribution is assumed over the entire electrode surface up to the
very edge of the electrode, where the current density increases abruptly,
which is quite close to the real state described by other authors2-5. The
illustration of this effect will be given in Section 4.2.2.
4.1.4

The depth of the penetration of a current line between the
electrode edges and the cell side walls

4.1.4.1 Mathematical model
Equation 4.12 can be rewritten in the form

and jâ&#x20AC;&#x2122; can be majorized by

giving:

110

Chapter 4

as the maximum length of a current line. U – E in Eqs. 4.22 and 4.23 is the
ohmic potential drop, but it can be substituted by the cell potential due to the
following facts.
The current along each line should be very low, and because of this the
electrochemical overpotentials at the edges of electrodes due to one current
line can be neglected relative to the ohmic potential drop. Hence, the cell
potential transforms into the ohmic potential drop along each current line
and U – E in Eq. 4.23 can be substituted by the cell potential from Eq. 4.19.
Substitution of U – E in Eq. 4.23 by the cell voltage from Eq. 4.19 gives:

or after rearrangement:

Assuming a linear approximation of the propagation of a current line the
relation between L’, the maximum depth of the propagation of a current line
penetration in the space between the edges of the electrodes and the cell side
walls, l and l’ is given by:

Substituting l’ from Eq. 4.25 into Eq. 4.26 and rearranging gives:

It can be shown that if:

4. The Current Distribution in Electrochemical Cells

111

then
and
and in the opposite case
and
This shows that the ability of an electrolyte to distribute the current density
uniformly increases with decreasing
product, i.e., with decreasing ohmic
polarization. Furthermore, is to be expected that the larger the spacing (the
distance between the polarization j – U curves for L = 0 and L > 0), the
worse current density distribution becomes.
4.1.4.2 Cell voltage-current density dependencies
Taking into account Eqn. 4.5 and 4.7 for L > 0 (assuming that the current
density in the homogenous field is equal to the overall one, i.e. that current
density distribution is uniform over all the electrode, except for the very
edge), Eq 4.19 can be rewritten in the form

If L’< L, L in Eq. 4.29 should be substituted by L’.

Hence, calculations of the current density-cell voltage dependencies must
begin with the determination of the relation between L and L’.
It is obvious that if:

electrolysis is predominantly under electrochemical control, and if:

the electrolysis is predominantly under ohmic control.
In the first case an S-shaped polarization curve can be expected, and a
straight line in the second one. Hence, the shape of the polarization curve is a
good indicator of the nature of the electrolysis process. On the other hand, the
degree of current line penetration into the solution between the edges of the

112

Chapter 4

electrodes and the side walls of the cell is an indicator of the ability of an
electrolyte to distribute the current density uniformly over the entire electrode.
The experiments were carried out in the cell presented in Fig. 4.6

The dependences of the current density on the cell voltage for different
interelectrode distances and different distances between the edge of the
electrode and side wall, for the system
at
a temperature of 20Â° C are shown in Figs. 4.7 â&#x20AC;&#x201C; 4.10.

4. The Current Distribution in Electrochemical Cells

113

114

Chapter 4

From Figs. 4.7-4.10, the change in the shape of the j-U dependencies as
the inter-electrode distance increases can be seen. In the region of lower
current densities, at the shortest interelectrode distance, the system is under
mixed activation-ohmic control. At higher current densities, although a concentration overvolatge appears, the control becomes ohmic because the
magnitude of the overvoltages is negligible in comparison to the ohmic
voltage drop in the electrolyte. As the interelectrode distance increases, the
electrolyte resistance in the cell increases too, giving rise to an increase in
the contribution of ohmic control of the system which in turn causes a larger
spreading out of the current lines between the edges of the electrodes and the
side walls. Therefore, the difference between the j-U curves for different
distances between the side wall and the edge of the electrode becomes larger,
meaning a worse current density distribution. At very large current densities,
approaching the limiting current, the contribution of the concentration
overvoltage to the total cell voltage becomes significant and the system is
under mixed diffusion-ohmic control. Finally, in the region of limiting
current densities, the system is entirely under diffusion control. The polarisation curves for different distances between the edge of the electrode and
the side wall are very similar indicating a minimal spreading of the current
lines out of the homogeneous electric field.

4. The Current Distribution in Electrochemical Cells

115

4.1.4.3
Determination of the current density distribution
The effect of the geometry of a cell on the ability of the electrolyte to
distribute the current density uniformly can be illustrated by plotting the
ratios of the current density in cells with different interelectrode distance l
(20, 50, 100 and 150 mm) and an electrode edge â&#x20AC;&#x201C; cell side wall distance L =
150 mm, and in cells with different electrode edge-side wall distance L
(12.5, 25, 50, 100, 150 mm) and an interelectrode distance l=150 mm to the
current density in a cell with the same l values and L = 0, as a function of
current density in a cell with L = 0, normalized to the limiting diffusion
current density, as shown in Figs. 4.11 and 4.12. It should be noted that the
increase of the limiting diffusion current density in a cells with L>0 over the
value in a cell with L=0 was not taken into account in the derivation of the
curves in Figs. 4.11-4.13.

Obviously, the larger the current density ratio, the lower is the ability of
the electrolyte to distribute homogeneously the current in the cell.
Hence, increasing the interelectrode distance leads to a worsening of the
current density distribution, as does increasing the electrode edges â&#x20AC;&#x201C; cell side
wall distance.

116

Chapter 4

4. The Current Distribution in Electrochemical Cells

117

The effects of the supporting electrolyte and the depositing ion concentration, insoluble anode and temperature on the ability of an electrolyte to
distribute homogeneously the current density are illustrated in Fig. 4.13.
All the above facts can be explained by discussing L', the depth of the
current line penetration between the electrode edges and cell side wall for
L'<L. Previously it was shown that:

and

For one and the same current density in a cell, the electrochemical part of
the cell voltage does not depend on the interelectrode distance, but the ohmic
drop in a homogeneous field is strongly dependant on it, which leads to an
increase of L' with increasing l. This produces decrease of the cell voltage at
a fixed current density, because of the decrease of the overall ohmic
resistance. Hence, at a fixed cell voltage, increasing L' will results in an
increase of the current density relative to a cell with L = 0 and, hence, a
worse current density distribution. The same will happen with increasing L
at l = 150 mm if the condition L' < L is not satisfied.
In the similar way, in the cells with the same l = 150 mm and L = 150
mm, L' will depend on the fractional contribution of the ohmic drop
to the
cell voltage. At a fixed l, the value of the product
increases faster with
increasing j than the electrochemical part of cell voltage. Hence, increasing
the depositing ion concentration and the stirring rate will cause the ability of
the electrolyte to distribute the current density uniformly worsen, as well as a
decreasing of supporting electrolyte concentration. Increasing value of the
Tafel slope and, probably decreasing exchange current densities will
improve the current distribution.
Increasing the temperature also has a significant effect, regardless of the
simultaneous decrease in and increase in j. This means that the decrease in
is more pronounced than the increase in j. In all cases, the increase in the
degree of diffusion control leads to a better current density distribution on a
macroprofile. On the other hand, in such a situation the current density
distribution worsens on a microprofile (see section 3.2).

118
4.1.5

Chapter 4
Quantitative treatment

Using data from Figs. 4.7-4.10, for L = 0 but with different value of l, the
cell voltage â&#x20AC;&#x201C; interelectrode distance dependencies for different current
densities can be obtained. The results of these calculations are shown in Fig.
4.14.

Obviously, the intercepts represent the difference of the anodic and the
cathodic overpotentials for selected values of the current density, and the
resistivity of the electrolyte can be determined from corresponding slopes.
The obtained values are given in Table 4.1.

4. The Current Distribution in Electrochemical Cells
4.1.5.1

119

Calculation of the cell voltage-current density distribution
dependences
The diagrams from Figs. 4.7-4.10, can also be presented in the form shown
in Figs. 4.15-4.18.

120

Chapter 4

4. The Current Distribution in Electrochemical Cells

121

It can be seen from Figs. 4.15-4.18 that the change of current with
increasing L virtually ends at L = l/2 for l < 2A, at L = A for l > 2A and that L'
can be successfully calculated using Eqn. 4.27 and the values from Table
4.1. Calculated Lâ&#x20AC;&#x2122; is marked on the j-L dependences in Figs. 4.15-4.18 by a
thin vertical line.
Hence, the Eq. 4.27 is valid for
and L > L'. At larger interelectrode
distances for l > 2A, L' remains constant and equal A.
Using equations 4.27 and 4.29, together with the
and values from
Table 4.1, dependencies like those from Figs. 4.7-4.10. were calculated and
the results are presented in Figs. 4.19 and 4.20.

It can be seen from Figs. 4.19 and 4.20 that the agreement between the
calculated and the measured values is very good. In this way the method for
the calculation of L' is verified, as is the possibility of the calculation of cell
voltage â&#x20AC;&#x201C; current density dependences.
Hence, the current density distribution in cells with plane parallel
electrodes can be calculated without experimental measurements by using
the data from simple polarization measurements, or literature data for kinetic
parameters.

122

Chapter 4

4.1.5.2

The critical current density for dendritic growth initiation at
the edges
The polarization curve equation is given by:

for
given by

The critical overpotential for dendritic growth initiation,

Substitution of
produces:

is

from Eq. 3.60 into Eq. 2.28 and further rearranging

where is the critical current density for the dendritic growth initiation. On
the other hand the edge current density is given by:

4. The Current Distribution in Electrochemical Cells

123

where

and

If L > Lâ&#x20AC;&#x2122;, the edge current density could be obtained by combining Eqs.
4.13, 4.14 and 4.25 as:

Assuming that maximum edge current density is given by Eq. 4.32 the
substitution of in Eq. 4.33 instead of and further rearranging produce:

from which the maximum current density,
in the homogenous field at
which dendrites at the edges do not grow can be calculated. It follows from
Eq. 4.34 that for:

and if

being in both cases larger than the current density corresponding to the end
of the Tafel linearity, which is the optimum current density for the
deposition of compact metal (see section 3.2.1.3.1). Hence, if deposition
current density corresponds to the end of the Tafel linearity dendrites will
not grow at the edges of the electrode. It should be noted that in metal

124

Chapter 4

electrorefinning working current density can be determined relative to the
initial concentration of depositing ions, because it remains constant or
increases during refining process. In electrowinning processes the working
current density must be determined relative to the final concentration of
depositing ions, because it is lower than initial one. The same reasoning is
valid in the case of L <Lâ&#x20AC;&#x2122;, meaning, in general, that if the current density in
cell is lower than
the dendrites and probably carrot like protrusion on
the electrode edges can not grow.

4.2

CELLS WITH LOW ANODE POLARISATION

Cells with a small cathode and a large anode are often used in
electroplating technology. In this case, a homogenous distribution of the
deposit over the entire cathode is required.
4.2.1

The dependence of the current density at the tip of a
stationary wire electrode on the current density in the middle
of the electrode

It can be seen from Fig. 4.21 that the dissipation of current lines from the
tip of a stationary wire electrode is more pronounced than in the case of the
edges of plane parallel electrodes. This is because the dissipation in the
former case occurs through the space, while in the latter case it takes place in
one plane, normal to the electrodes, to which two symmetrically positioned
points belong. Hence, it can be taken that the overall resistance between the
tip of the cathode and anode will be equal to an infinitely large number of
resistances, as in the case of the edges of two plane parallel electrodes
connected in parallel, being equal to zero.
The cell voltage, U, for the part of the system where the current density is
homogeneously distributed is given by:

where R is the ohmic resistance of the electrolyte and I current in the cell,
and for the tip of wire electrode:

where
and
are the overpotential at the tip of wire electrode and at the
edge of the cylindrical anode, respectively, or after elimination of U-E from
Eqs. 4.37 and 4.38

4. The Current Distribution in Electrochemical Cells

125

In this way the ohmic potential drop in a homogenous field transforms
into the electrochemical overpotential for points at the tip of a wire electrode
or in a similar position. This means a larger tip current density than in a
homogenous field. Finally, if the anode surface area is much larger than that
of the cathode i.e., if:

Eq 4.38 can be rewritten in the form:

The ability of an electrode to distribute uniformly current density on a
whole cathode can be easily estimated by comparing the cathodic
polarization curve with the cathodic current density-cell voltage dependance.
The lower is the difference between them, the better distribution of the
current density should be expected.
On the other hand, Eq. 4.41 can be rewritten in the form:

126

Chapter 4

assuming that in mixed control deposition:

or in activation controlled deposition:

where

It can be taken to the first approximation that the same relation is valid
also for the edge of a small, square stationary vertical cathode placed in the
middle of a large cylindrical anode because the current line distribution is
similar to the one from tip of wire electrode.
It follows that for
Eq. 4.42 can be rewritten in the form:

This means that the lower the resistance of the electrolyte the is lower the
difference in the current densities in the middle and at the tip of the
electrode. The increase of leads to a more uniform current distribution,
also. This can happen in the presence of strongly adsorbed species or during
deposition from some complex salt solution.
It is also seen that current density distribution on a macroprofile becomes
uniform if
but in this case rough and dendritic deposit appears, being
unuseful for plating purpose.
The difference,
between the current density at the tip (edge) and in the
middle of the electrode is obviously:

4. The Current Distribution in Electrochemical Cells

127

and

It follows from Eq. 4.42 that the exchange current density does not effect
the current distribution, but there is an exception in the cases of
electrodeposition processes characterized by very large value of the
exchange current density. When the deposition can be under diffusion or the
ohmic control and the deposit will be formed only at the edges or in a similar
position, where ohmic resistance is low. Decreasing the exchange current
density by complexing the depositing ion leads to a more homogenous
distribution. The above discussion is illustrated by the following examples.
4.2.2

Experimental evidence

4.2.2.1
The effect of ohmic resistance
In order to illustrate the effect of the ohmic resistance of a cell, deposition
of copper was performed at room temperature on to a stationary copper wire
electrodes (length 40 mm and diameter 0.8 mm) placed in the middle of a
cylindrical cell (length 5 cm and diameter 6 cm). The surface of the cell was
covered by a high purity copper plate from electrolytes containing
and
in
The plot of
cell voltage and overpotential vs. current density for the copper deposition
are shown in Figs. 4.22 and 4.23.
The relation between the current density at the electrode tip and in the
homogenous field can be estimated by considering the cathodic overpotential
â&#x20AC;&#x201C; current density and cell voltage â&#x20AC;&#x201C;current density dependencies. According
to Eq. 4.41, the tip overpotential is equal to the cell voltage for a wire
electrode.
It could be seen that the tip overpotential (cell potential) is larger than the
overpotential in the middle of the electrode during deposition from
while during deposition from
in
the overpotentials are practically the same. In the former case
there is a large difference in the morphology of the deposit at the tip and the
rest of the electrode (Fig. 4.24a), while in the latter case the quality of the
deposit is the same over the whole surface (Fig. 4.24b).

128

Chapter 4

4. The Current Distribution in Electrochemical Cells

129

This is in perfect agreement with Eq. 4.44.
4.2.2.2

Deposition in the presence of strongly adsorbed organic
additives (effect of increased cathodic Tafel slope)
In order to illustrate the effect of a strongly adsorbed organic additive16,
cadmium was deposited onto a stationary vertical flat copper electrode of
area 1 cm x 1 cm placed in the middle of a cylindrical cell of diameter 6 cm,
and height 5 cm. The cell surface was covered by the anode, which was
made from high purity cadmium plate. The reference electrode was a high
purity cadmium wire. The electrolyte used in all the experiments was a
solution of
in
to which
polyoxyethylene alkylphenol (9.5 mol ethylene oxide) was added.
The overpotential-current density and cell voltage-current density plots
for cadmium deposition are presented in Fig. 4.25. The cathodic polarization
curve obtained from potentiostatic polarization measurements has a similar
shape to that found for an anode which can become passive; above a certain
overpotential, increasing the cathode polarization leads to a decrease in the
cathodic current followed by a range of potential in which the overpotential
has little effect on the current. Current oscillations were observed at the
beginning of this plateau in some cases (see also section 3.1.6.2).
It was shown in a section 3.2.1.3.1 that the optimum plating overpotential
is determined by the upper limit of validity of the Tafel equation for the
deposition process. In this case, as can be seen from Fig. 3.13 that the
optimum deposition overpotentials for cadmium deposition are about 40 and
530 mV in the absence and in the presence of additive, respectively.

130

Chapter 4

Figure 4.25 shows that there is a large difference between the deposition
overpotential and the cell voltage (tip overpotential) at low overpotentials
which becomes negligible at high overpotentials, indicating a uniform current
density distribution16, due to the additive adsorption, as illustrated in Fig 4.26.

4. The Current Distribution in Electrochemical Cells
4.2.2.3

131

Deposition from a complex salt solution (effect of exchange
current density)
In order to illustrate the effect of deposition from complex salt solutions
silver was deposited from simple and complex salt solutions17. The
electrolytes used throughout the experiments were
in
solution and
in
to which was added ammonium hydroxide to dissolve the precipitate of silver
sulfate. The conductivities of the above solutions were almost the same. Silver
was deposited onto a stationary vertical platinum cathode (1 cm x 1 cm)
placed in the middle of a cylindrical cell (diameter, 6 cm, height 5 cm); the
surface of the cell was covered by the anode, a high purity silver plate.
Polarization curves were obtained for the platinum wire cathode, which had
previously been plated with silver from the ammonium complex salt solution.
The overpotential-apparent current density and cell voltage (edge
overpotential)-apparent current density plots for silver deposition from the
nitrate solution and from the ammonium salt solution are presented in Figs.
4.27 and 4.28, respectively.
Silver deposition from the nitrate bath is under pure diffusion control at
all overpotentials because
and at
the nucleation in the middle
of the electrode does not occur because of ohmic control before nucleation.
Hence, deposition from the nitrate bath is expected only at the edges where
ohmic resistance is better, as predicted by Fig. 4.27. This is illustrated in Fig.
4.29a and b.

132

Chapter 4

For the ammonium bath there is a region where deposition is under
activation control because
and a slope of
Hence, nucleation occurs over all surface. For deposition from the
ammonium bath, as predicted by Fig. 4.28, a more homogenous distribution
of the deposit is obtained, which is illustrated in Figs. 4.29c and 29d.

In this way the effects of the deposition process parameters, ohmic
resistivity on the current distribution are shown. It is obvious that decrease of
the ohmic resistance and the increase of cathodic Tafel slope improve the
current density distribution and in the case of
the decrease of to the
value
The effect of cell geometry and deposition conditions was
treated in the previous section.

4. The Current Distribution in Electrochemical Cells

4.3

133

CORNER WEAKNESS PHENOMENA IN
ELECTROFORMING

â&#x20AC;&#x153;Corner weaknessâ&#x20AC;? occurs in heavy deposits or electroforms at screened
cathode parts i.e. corners. The deposit is thinner and at these areas,, in extreme
cases, there is no deposition at all along the line of the corner bisector18.

The consequence is the emergence of fracture under negligible load along
the line of the corner bisection, instead of fracture at much higher loads across
the narrowest cross-section of an electroform normal to the line of pull.
To the best of our knowledge, a theoretical analysis of this phenomenon
has not been reported so far. The purpose of this work was to undertake one,
using the following assumptions19:
the potential difference between each of two points on the anode and
cathode is equal to the cell voltage,
the current lines are normal to the electrode surface,
along each current line a corresponding ohmic resistance exists and the
current lines are independent of and insulated from each other,
current lines in the vicinity of a protrusion divide into components
which are normal to the electrode surface and
the Kirchoff laws are valid for current lines branching.
4.3.1

Ohmic controlled deposition

According to the assumed model of current line division it follows that
there is no deposition along the line of bisection if the division of the current
lines occurs along the line indicated by the dashed line in Fig. 4.31. It can be

134

Chapter 4

seen that this configuration provides the same density of current lines at the
cathode as at the anode.

The ohmic potential drops along the current lines j and
Eqs. 4.47 and 4.48 respectively, asuming E=0.

are given by

where U is the cell voltage and is the resistivity of the solution. The ohmic
resistance along the current line is somewhat different. It consists of the
resistance between the anode and the dividing point (DP) and two
resistances, proportional to x and h â&#x20AC;&#x201C; x connected in parallel between the DP
and the cathode. Hence, the ohmic potential drop along current line can be
written as:

Elimination of U from Eqs. 4.47 and 4.49 and further rearrangement
gives

4. The Current Distribution in Electrochemical Cells
The current densities

and

135

are given by

and

Substitution of from Eq. 4.50 into Eq. 4.51 and 4.52 and further
rearrangement with use of Eqn. 4.47 gives

and

which enable the calculation of the deposit profiles at the cathode
represented by Fig. 4.31
The proposed model implies that there is no current component in the
direction of the corner vertex, and that the appearance of a crack along the
corner bisector is to be expected.
A compact deposit cannot be obtained directly, but rather by the build up
of the deposit in the x and y direction. An overlap of the x and y oriented
deposits should occur when the current density virtually does not depend on
the distance from the very corner. However, if the current density decreases
upon approaching the corner vertex, the deposits would not overlap and a
flaw would be created.
Equations 4.53 and 4.54 may be utilized for the calculation of the current
density distribution at the beginning of deposition. The results of this
calculation are shown in Fig. 4.32, as well as for different deposition times.
It is obvious that there should be no overlap of the deposit upon prolonged
deposition. Moreover, it should be noted that the profiles were calculated
assuming a constant current density, which is not the case in a real system
where the space in the vicinity of the corner vertex is increasingly screened as
the deposit grows. This implies that the real distribution of the metal deposit in
the corners is worse than that calculated and shown in Fig. 4.32.

136

Chapter 4

A number of microphotographs of deposit cross-sections illustrating the
"corner weakness" effect can be found in the literature. They are schematically exemplified by Fig. 4.30. It can be seen that the calculated deposit
profile (Fig. 4.32), with a crack appearing along the corner bisector, looks
very similar to that typically obtained in plating practice (Fig. 4.30).
4.3.2

Mixed activation - diffusion - ohmic controlled deposition

In this case, it is not possible to perform an analysis of the current density
distribution in the same manner as in the case of ohmic controlled deposition. Only a numerical solution can be obtained, regardless of the fact that
the current line distribution is the same as shown in Fig. 4.31.
It is to be noted that in the model used anode and cathode are not
isopotentional. Because of this the overpotentials in different points are
calculated by direct using of Eqs.2.39 and 2.44 and corresponding current
densities. It is assumed that cathode deposition is under mixed activation
diffusion control and that anode dissolution is under activation control.
In this case, the first step is the calculation of the voltage current curves,
for interelectrode distances l and l + h, using Eqs 4.55 and 4.56.

4. The Current Distribution in Electrochemical Cells

137

where U is the cell voltage, and and are the anodic and cathodic Tafel
slopes and exchange current density respectively and is the limiting diffusion
current density for the cathodic process, assuming the zero value of the
reversible potential difference. The corresponding cell voltage-current curves
shown in Fig. 4.33 enable the determination of the current densities on the
frontal parts of the cathode (j and ) for any cell voltage.

The overall current density along the current line from the anode to the
DP is obviously the sum of the partial ones branching at the DP, i.e.
Hence, Eqs. 4.57 and 4.58 are valid

Elimination of U from Eqs. 4.57 and 4.58 gives

138

Chapter 4

Obviously, both sides of Eq. 4.59 represent the potential drop between
the DP and the cathode,
which can be plotted as a function of either
or as shown in Fig. 4.34a and 4.34b, respectively.

From the dependencies shown in Fig. 4.34, the corresponding values of
and can be extracted by interpolation, for any
and x. These are
required for the calculation of the potential drop between the anode and the
DP,
according to Eq. 4.60 and the overall cell voltage, U, according to
Eq. 4.61.

Now, U, and can be tabulated or plotted as functions of the overall
current density,
as exemplified in Fig. 4.35, for one value of x.
The situation is similar in the case of mixed diffusion-activation control.
The profiles of the deposit at different times (Fig. 4.36) were calculated
according to the procedures given above and the corresponding parameters:
and
h=10 cm and l=5 cm.

4. The Current Distribution in Electrochemical Cells

139

Comparing the calculated deposit profiles for pure ohmic control (Fig.
4.32) with those pertaining to diffusion-activation control (Fig. 4.36), a
significant difference in the cross section can be noticed. In the latter case,
despite a much poorer system geometry (cf. h to l ratio) from the viewpoint
of current distribution, no failure in the deposit appears. Even so, a flaw in
the deposit along the corner bisector may be expected in both cases.

140
4.3.3

Chapter 4
Activation - diffusion controlled deposition

In this case, the current line between the DP and the cathode splits into
two equal parts. Hence, for
one can write:

and simultaneously:

Elimination of U from Eqs. 4.62 and 4.63 permits the correlation between
j and in the form of Eq. 4.64, which cannot be solved explicitly if

If

and

Eq. 4.64 can be rewritten in the form:

or

which holds for pure activation control.
Evenly distributed deposits, without "comer weakness", may be obtained
only by deposition under complete activation control at high Tafel slopes
(Fig. 4.37). In practice, this is usually achieved by employing appropriate
surface-active additives, as seen in Fig. 4.38.
In this way not only is the â&#x20AC;&#x153;corner weaknessâ&#x20AC;? effect fully explained, but
also a new method of current density distribution evaluation in
electrochemical cells is promoted.

4. The Current Distribution in Electrochemical Cells

4.4

141

CONCLUSIONS

The current density distribution in electrochemical cell was considered in
a new manner on the basis of equation of the electrode kinetics. In this way
it was possible to illustrate the effect of deposition conditions, geometry of
the system and kinetic parameters of deposition processes. Besides, three
main problems of current density distribution in electrometallurgy were
treated semiquantitatively. The edge effect in metal electrorefinning and
electrowinning processes was discussed in details and it was shown that the

142

Chapter 4

dendritic growth at the edges of the electrodes can be avoided by keeping the
deposition current density below some critical value, probably little larger
than this corresponding to the end of the Tafel linearity. At this current
density the less coarse deposit in the homogenous field without the dendrites
at the edges of electrodes can be expected.
The current density distribution in electroplating and electroforming were
also treated semiquantitatively and it was shown that decrease of ohmic
resistance of electrolyte and increase of the Tafel slope for the cathodic
process improves it. Obviously, all above discussion is valid if the local
values of limiting diffusion current density do not varies along electrode
surface, i.e. if the effect of the hydrodynamics can be neglected. As a metter
of fact, the diffusion layer thickness may vary along the electrode interface
due to hydrodynamic conditions and cause the different deposition
conditions. This phenomenon is treated elsewhere20,21

It has been known for a relatively long time that the application of a
periodically changing current in metal electrodeposition practice leads to
improvements in the quality of electrodeposits. Three types of current
variation have been found useful: reversing current (RC); pulsating current
(PC); and sinusoidal, alternating current superimposed on a direct current
In recent years, the beneficial effects of pulsating overpotential
(PO) have also been discussed3. Even though this kind of electrodeposition
at a periodically changing rate (EPCR) is important from a theoretical point
of view and offers a variety of experimental possibilities, it is as yet not
frequently used in metal electrodeposition practice.
5.1.1

Reversing current

Reversing current is represented schematically in Fig. 5.1. It is
characterized by the cathodic current density,
and the anodic current
density,
as well as by the duration of flow of the current in the cathodic
and the anodic direction, and respectively. Naturally,

where T is the full period of the RC wave.
145

146

Chapter 5

The average current density is then given by:

and for

where

RC is used in the second and millisecond range7.
5.1.2

Pulsating current

Pulsating current consists of a periodic repetition of square pulses. It is
similar in shape to RC except for the absence of the anodic component, as is
shown in Fig. 5.2. PC is characterized by the amplitude of the cathodic
current, the cathodic deposition time, (on period), and the time interval
in which the system relaxes (off period).

5. Electrodeposition at a Periodically Changing Rate

147

The full period, T, is given by

and the average current density by

or

where

It should be noted that rectified sinusoidal AC, especially half-rectified
sinusoidal AC, often termed pulsating current in the literature, shows similar
effects to those of PC7.

148
5.1.3

Chapter 5
Alternating current superimposed on direct current

Sinusoidal AC superimposed on a direct cathodic current (DC) is
represented in Fig. 5.3. It is characterized by
and the frequency, which
is usually 50 or 60 Hz. The resultant is termed an asymmetric sinusoidal
current. The average current is equal to
At a given DC value, three different types of current can be obtained,
which can be denoted as follows:
"rippling current";
â&#x20AC;&#x153;pulsating current";
"current with an anodic component." The last
type is mainly used in plating practice.

5.1.4

Pulsating overpotential

Pulsating overpotential consists of a periodic repetition of overpotential
pulses of different shapes. Square-wave PO is defined in the same way as PC
except that the overpotential pulsates between the amplitude value
and
zero instead of current density. Non-rectangular pulsating overpotential is
defined by the amplitude of the overpotential,
frequency, and overpoten7
tial waveform .
There are a number of different current and overpotential waveforms
used in EPCR12,13, but the most important have been mentioned above.

5. Electrodeposition at a Periodically Changing Rate

149

5.2

SURFACE CONCENTRATION OF DEPOSITING
IONS IN THE PERIODIC CONDITION

5.2.1

Electrodeposition with periodically changing range in the
millisecond range

Electrodeposition with a periodically changing rate can be described in
terms of time-and distance-dependent concentrations:

Equations 5.9 to 5.12 are solved for different j(t) shapes and the solutions
applied to different types of problems7.
The current density j(t) is the periodic function of time, which for
periodic reverse currents14 is given by:

for pulsating currents15 by:

and for AC superimposed on DC16 by:

150

Chapter 5

In the case of pulsating overpotential3, j(t) is given by:

The surface concentration under periodic conditions can be evaluated as
follows. For j(t) given by Eq. 5.13, the solution of Eqs. 5.9 to 5.12 for x = 0,
t = [m + 1(r + 1)] T, and
i.e., at the end of the cathodic pulses, under
the periodic conditions is given by14:

where
The surface concentration,
at the end of the anodic pulses under the
same conditions, i.e., for x = 0, t = (m + 1) T, and
is given by:

It is easy to show that for a sufficiently long period T,
the system behaves as under DC conditions. For
taking into account Eq. 2.29, Eqs. 5.17 and 5.18 become

where
and

5. Electrodeposition at a Periodically Changing Rate

151

and

For a sufficiently small value of T,

For j(t) given by Eq. 5.14, solution of Eqs.5.9 to 5.12 for x = 0, t=[m +
1/(p + 1)] T, and
i.e., at the end of the cathodic pulses under periodic
15
conditions, is given by :

The surface concentration at the end of pauses,
conditions [x = 0, (m + 1) T,
is given by:

As in the previous case for
conditions where

the system behaves as under DC

and

For

under the same

it follows from Eqs. 5.22 and 5.23 that

152

Chapter 5

It is obvious from Eqs. 5.2, 5.4, and 5.7 and Eqs. 5.21 and 5.26 that, in
both cases,

taking into account also Eq. 2.29
For j(t) given by Eq. 5.15, the surface concentration under periodic
conditions is approximately given by16

Hence, at sufficiently small value of T and not extremely high values of
in AC will also be given by Eq. 5.27, implying that at sufficiently high
frequencies, surface concentration is determined by the average current
density regardless of the shape of the current wave.
In the case of a rectangular pulsating overpotential,
as a function of
time, is given by17:

Assuming that the surface concentration is determined by the average
current density, Eq. 5.16 can be rewritten in the form

For a sufficiently high value of
periods to:

and during the off periods to:

Eq. 5.30 reduces during the on

5. Electrodeposition at a Periodically Changing Rate

153

The average current in PO deposition can easily be determined by

The overpotential amplitude is then given by

and

where

and

Polarization curves for the average values for the copper deposition
have been successfully calculated from the stationary polarization curve
using Eq. 5.3517 for
This is a good evidence that in PO deposition,
the average current density also determines the surface concentration of the
depositing ion.
The overpotential amplitude is larger than in the DC regime for one and
the same average current density. At the same time, the diffusion overpotential remains constant, depending on the average current density only.
Hence, the part of activation control in the overall amplitude overpotential
increases with increasing pause to pulse ratio.
The situation is similar in pulsating or reversing current electrodeposition.

154
5.2.2

Chapter 5
Capacitance effects

From the above discussion, it can be concluded that the useful range of
frequencies is limited by mass-transfer effects at low frequencies. At high
frequencies, the useful range is limited by the effect of the capacitance of
the electrical double layer15. This is shown here for PC deposition. The
time dependencies of the overpotential during the current pulses are shown
in Fig. 5.4.

Mass-transfer limitations cause an increase of the overpotential at
deposition times longer than the transition time as shown in Fig. 5.4a; the
system enters full diffusion control at low frequencies if
This is
10,15
followed by an increase in the average overpotential . At high
frequencies, the PC is used both for double-layer charging and discharging,
and for the deposition process, as illustrated by Fig. 5.5. The capacitance
current during periodic charging and discharging of the double layer, at

5. Electrodeposition at a Periodically Changing Rate

155

frequencies at which the effect of the double layer cannot be neglected,
produces a smearing effect on the Faradic current wave, as illustrated by
Fig. 5.4b.

Hence, as the frequency increases, the faradic current wave flattens,
approaching a DC shape, and gives the same quality of deposit as DC even
though the overall current appears to be pulsating. This is also followed by
an increase in the average overpotential. Hence, the minimum average
overpotential is a good indicator of the optimum frequency range of
pulsation3,10,15 in PC deposition. This range depends on the average current
density and p, but, in general, the frequency lies in the range between 10 and
100 Hz, as illustrated in Figs. 5.4c and 5.4d.
In PO deposition, the effect of the double layer capacitance becomes less
pronounced at higher frequencies compared to the other cases10. Also, at
very high frequencies, the shape of the PO wave changes; for example, a
square-wave PO becomes similar to a triangular one10,17.

156
5.2.3

Chapter 5
Reversing current in the second range

For T close to the behaviour of the system under RC conditions has to
be analyzed using Eq. 5.187. In this case, the concentration distribution
inside the diffusion layer at the end of the anodic pulse is close to that given
by Eq. 5.10. It follows from Eq. 5.18 that this will occur at:

or

It is known14 that for
the series in Eq. 5.39 can be
approximated using only the first term (k = 0). Hence, for

or

It is easy to show that for T = 3
r = 0.7 and for T = 16 r = 0.2 by
assuming that Eq. 5.40 is valid for T > 3 and that for T > 16 the system
behaves as under DC conditions. The optimum ratio
is given by

for periods T such that

5. Electrodeposition at a Periodically Changing Rate

157

if
for
cm and
A good agreement between
the shape and the frequency of the RC calculated in this way and literature
data is obtained, because in practically all cases, according to Bakhvalov2,

On the other hand, the solution of Eqs. 5.9 to 5.12 for

is given by18

for

It follows from Eq. 5.46 that the surface concentration of depositing ions
can be given by:

The maximum amplitude of the current density variation,
corresponding to
after a deposition time is given by

It is obvious that using Eqs. 5.1 and 5.4 and r =f(T) given by Eq. 5.41,
Eq. 5.48 can be rewritten in the form

In this way the complete RC wave can be estimated, or precisely
calculated without approximations using a computer.

158

5.3

Chapter 5

PREVENTION OF THE FORMATION OF
SPONGY DEPOSITS AND THE EFFECT ON
DENDRITIC PARTICLES

EPCR is used in the charging of silver-zinc storage batteries, to prevent, or to
delay, the formation of spongy and dendritic deposits of zinc19,20. It is impossible
to obtain smooth deposits of zinc from alkaline zincate solutions during
prolonged deposition at a constant rate20,21, because at lower overpotentials,
spongy deposits arise while at higher overpotentials, dendritic zinc is formed.
It is well known that the reversible potential of a surface with radius of
curvature
would depart from that of a planar surface by the quantity22

where is the interfacial energy. The filaments which form spongy deposits
have extremely small tip radii. This makes the equilibrium potential of the
spongy deposit 7 to 10 mV more cathodic than that of zinc foil23,24.
Spongy deposit formation can, however, be completely prevented by PO
deposition20, as illustrated in Fig. 5.6.
Obviously, the more negative filaments dissolve faster during the off
period than the flat surface, resulting in a smooth deposit. This is also valid
for deposition using all current or overpotential waveforms that are
characterized by some anodic current flow3,7.
If spongy deposit formation is prevented or delayed the charging of the
silver -zinc batteries is considerably improved3.

5. Electrodeposition at a Periodically Changing Rate

159

The quantitative treatment of the selective dissolution during pauses can
be performed as follows25. Equation 5.30 is valid for a flat electrode surface
or protrusions with sufficiently large tip radii, where the surface energy term
can be neglected. If it can not be neglected in zinc deposition, Eq. 5.30. can
be rewritten for the tip of a protrusion inside the diffusion layer (h â&#x20AC;&#x201C; height
of protrusion and
tip radius) in the form:

where is the tip current density and
macroelectrode.
This is because:

is the average current density to the

if
or
( and
are the concentration of the depositing ions on
the electrode surface and on the tip of a protrusion, respectively), if
The output current during pauses
become

It is easy to show that the difference between the current density on the
tip of a dendrite and the flat surface during the "off â&#x20AC;? period can be given by

This means that the dissolution of the a protrusion with tip radius is faster
relative to the flat surface or a protrusion with a sufficiently large value of
It is obvious that spongy filaments can be completely (Fig. 5.6), and
dendrites with low tip radii partially or completely dissolved during the
pause, (Fig 5.7.). This means that both branching of dendrites and the
formation of agglomerates can be prevented in square-wave pulsating
overpotential deposition. In this way, even powder particle like that in Figs.
5.8 and 5.9 can be obtained.

160

Chapter 5

5. Electrodeposition at a Periodically Changing Rate

161

The monocrystal surfaces of silver powder particles can be explained by
the assumption that during the off period of PO the adatoms in nonstable
positions will dissolve faster than atoms in stable position in lattice. The
similar effect on the morphology of powder particles can be seen in RC
deposition28,29, which leads to the strong effect on the apparent density of
copper powders29.

5.4

COMPACT DEPOSITS

5.4.1

Surface film

The first stage of metal film formation is nucleation on a foreign
substrate. The nucleation rate, J, is given by Eq. 3.10 and depends strongly
on the deposition overpotential. The nucleation overpotential is larger than
the stationary one in galvanostatic deposition, and stationary values of the
overpotential can be used in discussions of the effect of EPCR in
galvanostatic as well as in potentiostatic deposition. It is obvious that in all
cases of EPCR, the overpotential amplitude,
is larger than in constantcurrent or constant overpotential deposition, for the same average current
density.30 Nucleation rate, J, in DC regime is given by:

Therefore,

162

where

Chapter 5

and

Hence, for the same current density:

as illustrated in Fig. 5.10.
On the other hand the increased amplitude on the current density leads to
an increase of the ohmic potential drop during the pulses in EPCR relative to
constant regimes and Eq. 3.5

can be rewritten in the form:

It follows from Eq. 3.5 and Eq. 5.58 that:

because with increasing p

Hence, the increasing nucleation density is also due to the decreasing
zero nucleation zone radii. This effect leads to an increased coverage of the
foreign substrate by the same quantity of deposited metal and to decreased
porosity, surface resistance and increased density of deposit. Also, it can be
expected that increase in compactness is associated with a decrease in
internal stresses and increased ductility and hardness of metal deposits7.

5. Electrodeposition at a Periodically Changing Rate

5.4.2

163

Electrode surface coarsening

The general equation of the polarization curve is given by:

for the flat part of an electrode and by

for the tip of a protrusion
be neglected.
Using Eq. 5.19 in the form

it is easy to show that

around which the lateral diffusion flux can

164

Chapter 5

and

if
where
and
are the surface concentration of depositing ions on the flat
electrode surface and on the tip of a protrusion respectively.
The rate of increase at the tip of a protrusion relative to the flat surface is
given by31

or

after substitution of
and
from Eqs.5.62 and 5.63 into Eq.
5.65a and further rearranging, assuming
It was shown earlier that at sufficiently high frequencies, the average
current density in electrodeposition at a periodically changing rate produces
the same concentration distribution inside the diffusion layer as a constant
current density of the same intensity. Hence, Eq. 5.65b is valid for all cases
of electrodeposition at a constant and periodically changing rate at
sufficiently high frequencies.
However, an increase in surface coarseness in deposition using a
rectangular pulsating overpotential or pulsating current is only possible
during the pulses of current or overpotential31 and the integral form of Eq.
5.65 can be written as

5. Electrodeposition at a Periodically Changing Rate

165

if

Equation 5.66 is valid for a pulsating current, square wave pulsating
overpotential and reversing current in the millisecond range under the
assumption that the entire surface dissolves uniformly during the pauses. The
deposits obtained by constant and pulsating overpotential in the mixed
control under other conditions are the same are shown in Fig 5.11. The
deposit obtained by pulsating overpotential is considerably less rough.
The copper deposits obtained under activation and mixed control as those
from Fig. 3.21 are shown in Fig 5.12. A considerable decrease in the grain
size of deposit obtained at low current densities (in the activation controlled
region Fig. 3.21a and Fig. 5.12a) due to the increase of the amplitude of the
overpotential relative to the corresponding value in constant overpotential
deposition, can be seen. There is no qualitative change, however, in the
structure of the deposit.
A qualitative change in the structure of the deposit appears in mixed
controlled deposition (Fig 3.21c and Fig. 5.12b). It is seen that the
protrusions caused by mass transport limitations are strongly reduced
relative to the deposits shown in Fig. 3.21c, but the grain size in enlarged. It
is obvious that the grains obtained by pulsating overpotential with current
densities belonging to the region of mixed control, Fig. 5.12b, are almost as
regular as those deposited under activation control (Fig. 5.12a). This is
obviously due to the increased degree of activation control during the
overpotential pulses and the increased grain size relative to those in Fig.
3.21b and c is due to the selective dissolution during the "offâ&#x20AC;? periods. The
smaller nuclei formed during the overpotential pulse will be completely or
partially dissolved during the overpotential pause and the current density and
the current density on the partially dissolved ones during the next
overpotential pulse will be considerably lower than on larger ones because of
their more negative reversible potentials, and the growth of larger grains will
be favorized.
In this way the appearance of the deposit shown in Fig. 3.21c changes
and becomes that shown in Fig. 5.12b which was formed using PO
deposition of the same quantity of deposited metal and average current
density. It can be also seen from Fig. 5.12 that a good deposit can be
obtained by PO deposition over a wide range of current densities. This
means that in EPCR deposition current density can be considerably
increased relative to DC case.

166

Chapter 5

On the other hand, it is known that the orientation of nuclei strongly
depends on the depositing overpotential and that the electrode reaction
parameters can be different for different crystal planes. It is therefore not
surprising that the effect of structure on EPCR has been reported for many
cases. In some cases, deposits which behave as monocrystals7 and deposits
with improved crystal perfection can be obtained.

5. Electrodeposition at a Periodically Changing Rate

167

It is obvious that the same reasoning is valid for RC in the millisecond
range and PC. Some different situations appear in the case of RC in the
second range32.
The surface concentration changes during the cathodic pulse in RC
deposition in the second range according to Eq. 5.46.

where

In this case

is also valid and substitution of

from Eq. 5.67 in Eq. 5.69 gives

for the tip of a protrusion and

for a position a flat surface. If all the surface is isopotential, elimination of
from Eqs. 5.70 and 5.71 and further rearranging produces

168

Chapter 5

The difference in the current densities at the tip of a protrusion and the
flat portion of the electrode is then given by:

for

Now, according to Eqs. 5.72 and 5.73 it can be written

or in the integral form:

where

for the first phase.
Assuming that the surface will dissolute uniformly during the anodic
the increase in the
period, it is obvious that because f(0) = 0 and
surface coarseness in the RC regime will be lower than in the DC regime
until the condition

is satisfied. The above reasoning is valid if a polycrystalline deposit is
obtained and the same derivation as in case of CO deposition can be used.
It can be seen from Fig. 5.13 that the structure of the deposit obtained by
RC in the second range is more similar to that obtained in the DC than in the
PC regime, but the surface coarseness of this deposit is considerably lower
than in DC case, being close to this in PC deposition.
This is because in RC deposition there is a considerable concentration
polarisation, producing polycrystalline deposit.

5. Electrodeposition at a Periodically Changing Rate

5.5

169

Current density and morphology distribution on a
macroprofile

The current density distribution on a macroprofile in EPCR has been
treated in several papers7. It seems that this distribution improves the
deposition by current waves with anodic flow but that without this flow it is
worse than in DC deposition. This behaviour can be successfully
explained34.
Assuming that Eq. 4.43 is valid for the flat electrode in a cell with low
anode polarization also, the current densities in the middle, j, and at the edge,
of a flat electrode in a cell with low anode polarization can be related by

if the deposition in both cases is under activation control in the Tafel region
and that the limiting diffusion current density is the same in the middle and
at the edge of the electrode, I is the cell current corresponding to j. During
the current pulses the amplitude values of current densities and current
should be substituted in Eq. 4.43 producing

where
and are the amplitude values of current densities in the middle
and at the edge of electrode and is the amplitude of the current in the cell.
On the other hand, the amplitude in pulsating current deposition and average
current density are related by Eq. 5.7

170

Chapter 5

so Eq. 5.78 can be rewritten in the form

assuming that

if

then
meaning worse current density distribution in PC than in DC
conditions
The effect of reversing current on the current distribution at the
macroprofile level can easily be discussed for the case of activationcontrolled deposition if the Tafel slopes of the anodic and cathodic processes
are different, as they are for copper deposition and dissolution in sulphate
solutions. With the assumption that the current density in RC deposition is
sufficiently high so that the effect of the opposing processes can be
neglected, the limiting diffusion current density is the same over all
electrode surface. The difference between the current density at the edge and
that at the middle of the electrode in cathodic deposition is

and for anodic case

since
for copper deposition, where and
corresponding to and It is obvious that for

are the cell currents

5. Electrodeposition at a Periodically Changing Rate

171

which occurs when

(if
and
deposits of equal thickness can be obtained at the
edge and at the middle of the electrode. In this way, a completely uniform
average current density distribution at the macroprofile level can be obtained
in RC deposition. The diffusion limitations of the cathodic processes will
improve the distribution in RC, but this approach is sufficient to explain the
essence of the effect, as illustrated in Fig. 5.1434.

The best current distribution is expected in the case of PO if all the
electrode surface can be taken as an isopotential. Under the assumption that
the limiting diffusion current density does not vary over the electrode surface
area, the same current density can be expected over all points of the
electrode. A good approximation of PO deposition can be the RC deposition
by the current wave optimized relative both current density distribution on
micro and macro profile as recently shown35. Hence, it seams that RC should
be the optimum regime of EPCR. Besides, the crack-free chromium deposits
with improved current density distribution on both micro and macroprofile
and with practically no reduced hardness were obtained recently36,37, by RC
deposition. This means that the formation of unstable chromium hydride can
also be prevented by RC, but this phenomenon has been not treated
semiquantitatively so far.

172

5.6

Chapter 5

CONCLUSIONS

The beneficial effect of EPCR in the electrodeposition of metals are
clearly demonstrated and explained in a semiquantitative way. Therefore, for
the same effective current density EPCR requires a somewhat larger energy
consumption7, than in DC deposition as well as the more expensive current
generators. This should not be considered as an important drawback of such
electrolysis, bearing in mind all the considerable improvements of the
process of metal deposition that result.
It appears that in each particular case one should compare all the
characteristic of both deposition with constant rate and EPCR, and then
decide which one to apply, considering all the requirements in relation to the
quality of the deposits, the productivity of the electrolyser, and other
parameters of the electrolytic production operation.

Electrowinning of a metal is based on the electrolysis of aqueous
solutions or melts of metal salts with insoluble anode1,2.
The basic reactions during electrowinning from aqueous solutions are2:
- cathodic deposition of the desired metal:

and
- oxygen evolution on the anode:

Electrowinning of a metal by the electrolysis is usually performed in an
aqueous solution of the sulfates of a metal in dilute sulfuric acid, with the
aim to avoid problems related with the deposition of the hydroxide of the
metal. Sulfuric acid is chosen because of its relatively low price2.
Several processes of preparations and enrichment of the metal ore proceed
the electrolysis, which is the final phase in the electrowinning process.
Bearing in mind the fact that metals are found in nature usually in the
form of their sulfide and oxide ores, the preparation usually consists in2:
I Enrichment of the ore, serves for the removal of the wastes by flotation
which leads to a concentrate of metal sulfides and/or metal oxides.
175

176

Chapter 6

II Roasting â&#x20AC;&#x201C; oxidation of the concentrate, if the dominate form of the
metal in the concentrate is the metal sulfide which is insoluble in sulfuric
acid, then the roasting is performed in the air with the aim of transforming
the sulfide into the oxide which is soluble in sulfuric acid.
III Leaching of the concentrate, is performed with dilute sulfuric acid,
impoverished electrolyte from the electrolysis process, which gives a
solution of the metal sulfate with some impurities.
IV Refining of the leach means the elimination of impurities with more
positive potentials, which could be deposited together with the metal, thus
decreasing its purity, or result in the hydrogen evolution reaction.
Elimination of the impurities is performed by increasing the pH of the
solution, by precipitation with hydrogen sulfide or by cementation.
V Electrolysis serves for the final extraction of the metal by cathodic
deposition in the electrochemical reactor. Since oxygen is evolved at the
anode according to Eq. 6.2, the concentration of metal ions decreases and the
acidity of the electrolyte increases, which can result in hydrogen evolution
together with metal deposition at the cathode. The compensation of the metal
ions concentration is performed in phase III.
Although practically all metals can be obtained by the electrochemical
procedure, the most wide spread technologies involve the electrowinning of
zinc from a solution of its sulfate1.

6.2

THEORETICAL ASPECTS OF ZINC
ELECTROWINNING

Apart from the conventional metallurgical procedure of zinc production,
today the most common procedure is the electrowinning of zinc by
electrolysis of an aqueous solution of zinc sulfate. This electrochemical
procedure gives zinc of 99.99 % purity and less loss of cadmium than the
classical pyrometallurgical treatment.
During the electrolysis of an aqueous zinc sulfate solution, the basic
electrode reactions are:
cathodic deposition of zinc

and oxygen evolution at the insoluble anode

6. Electrowining

177

Although the electrode potential of the reduction of zinc ions is much
more negative than the electrode potential of the reduction of protons (about
0.8 V), it is possible to deposit zinc from the acid solution with a high
current efficiency thanks to the extremely high value of the hydrogen
overpotential on zinc.
However, the presence of impurities (e.g metal ions with reduction
potentials more positive than that of protons) will cause these metals to be
deposited together with zinc. The results is the formation of cathodic
surfaces having a low value of the hydrogen overpotentials. Intensive
hydrogen evolution will occur on such surfaces together with the dissolution
(corrosion) of the already deposited zinc, following the mechanism of local
galvanic cells. Impurities such as copper, bismuth, germanium and antimony
do not just decrease the current efficiency, but completely prevent the
deposition of zinc. It can be concluded that the electrolyte for zinc
electrowinning has to be completely freed from such impurities before the
electrolysis. Hence, the basic aim of zinc ore treatment is the production of
pure zinc sulfate without impurities which would exhibit a bad influence on
the cathodic reaction2.
6.2.1

Technological scheme of zinc electrowinning

The composition of the zinc ore influences the manner of treatment of the
zinc sulfate solution. Zinc sulfate is extracted from an oxide ore after
removing the wastes, with the spent solution of zinc sulfate and sulfuric acid
from the electrolysis, according to the reaction2:

Extraction from a sulfide ore necessitates roasting in an air atmosphere,
during which the main reaction is the formation of zinc oxide according to:

Impurities in a zinc ore can be classified into three main groups3:
1. Fe, Si, Al, Sb, Bi, Ge, Sn
2. Cu, Cd, Ni, Co
3. Pb, Ag,
During the dissolution of zinc sulfate, according to Eq. 6.4, the extent of
the free acid decreases, which leads to an increase of pH of the solution. This
results in the precipitation of Fe(III), Al and partly Cu in the form of
hydroxides. Owing to the high specific surface of Fe(III) hydroxide, Sb, Bi

178

Chapter 6

and Ge are adsorbed on its surface and in that manner removed from the
solution, while As forms the insoluble
Fe(II) hydroxide
precipitates at pH 6.5, at which pH zinc(II) hydroxide precipitates too. For
this reason it is necessary to oxidize
to
This is achieved by the
addition of
which reacts with
according to the reaction:

and
also react with
at lower values of pH, giving the
corresponding three-valent ions which are deposited in the form of
hydroxides,
and NiOOH, and so in this way partly removed.
During the electrolysis
is formed by the anodic oxidation of
ions according to the reaction:

The impurities of the third group are insoluble in the leach.
The precipitate, containing impurities of the I, III, and partly of the II
group, are removed from the leach by filtration. The removal of group II
impurities is based on cementation of the metal ions from the solution with
zinc powder, according to the reaction:

After these treatments, the concentration of zinc ions varies between 120
and the concentration of the free acid between 150 and
Typical contents of metal ions in the electrolyte for the
electrowinning of zinc is given in Table 6.13.

and

It is common to add between 10 and
of glue which serves to
enhance the quality of the deposit and to avoid the formation of dendrites
which could lead to short circuits between the anode and cathode during
electrolysis, and also to a decrease in the current efficiency2.
The described procedures are schematically given in Fig. 6.1.
The cathode material used in the electrochemical cell is usually
aluminium sheet 4 to 8 mm in thickness3. Aluminium is used owing to the
poor adhesion of the deposited zinc, which can later be easily removed from
the cathode2. Lead sheet 5 to 10 mm in thickness, and in average, having the

6. Electrowining

179

dimensions of 100 x 600 mm, serves as the anode2,3. The life time of these
lead anodes is 1.5 – 2 years, but if the lead anode is alloyed with 1 – 2 % of
silver, the life time is increased to 3 – 4 years2.

The average electrolysis time is between 10 and 24 h. during which time the
concentration of zinc should drop to
After the electrolysis the
cathodes are taken from the cell and the zinc is mechanically removed.
Typical conditions for the electrowinning of zinc are as follows2,3,4:
the cathodic current density:
current efficiency:
88–92%
3.3–3.6V
voltage:
temperature of the electrolyte:
30–40°C
specific electrical energy consumption:

180

Chapter 6

For the electrowinning of 1 t of zinc in average about: 2.5 t of the
concentrate, 4 kg of
16 kg of Zn powder and 62 kg of sulfuric acid
are consumed2.
The zinc concentrate usually contains 0.5 â&#x20AC;&#x201C; 1 wt. % of Cd, which is why
in most zinc electrowinning plants, Cd is obtained parallel by a similar
process3,4.

Electrorefining of a metal by electrolysis is a way of purification of a
metal previously extracted by classical metallurgical or electrochemical
processes, with the aim of removing impurities, which could exhibit in
negative effects on the physico窶田hemical and mechanical properties of the
metal. In principle, almost all metals can be electrorefined, but judging by
the amount of 8 million tones per year, copper electrorefining outweighs all
others1,2.
The procedure of a metal electrorefining is schematically given in Fig.
7.1.

181

182
7.1.1

Chapter 7
Selectivity of metal dissolution and deposition

The main principle of electrorefining is based on the anodic dissolution
of a metal containing impurities and its cathodic deposition with an as much
as possible reduced amount of impurities. The selectivity principle of metal
dissolution and deposition can be explained starting from a metal M placed
in a suitable solution containing its ions
which leads to the
establishment of the equilibrium electrode potential
Under some
value of current density, j, the base metal is anodically dissolved

having an anodic potential

and cathodically deposited

having a cathodic potential

as shown in Fig. 7.22.

All metal-impurities having an equilibrium potential more positive than
(such is metal
) will not be anodically dissolved, but owing to the
dissolution of the base metal (erosion) they will be transferred into the

6. Electrowining

183

electrolyte and fall down to the bottom of the reactor forming the anodic
slime.
Metal-impurities which have an equilibrium potential more negative than
(such as:
and
will be dissolved together with the base metal
M, and transferred into the electrolyte in the form of metal ions. Exceptions
are those metals whose cations can react with the anions in the electrolyte
forming sparingly soluble salts which sediment as anodic slime2.
During cathodic deposition of a metal, all the metals whose equilibrium
potentials are more positive than the potential of the cathode
(metals
and
could be deposited, depending on their concentrations and
deposition overpotentials. In the example given in Fig. 7.2. metal
will be
anodically dissolved, but not cathodically deposited, while metal
will not
be anodically dissolved, but transformed into the anodic slime2.
It can be concluded that the selectivity principle of electrorefining is
based on such electrolysis conditions which ensure that metal impurities that
could be cathodically deposited will not be anodically dissolved (anodic
slime) and impurities that could be anodically dissolved will not be
cathodically deposited, but remain in the electrolyte.

7.2

THEORETICAL ASPECT OF COPPER
ELECTROREFINING

Copper produced by metallurgical processes contains significant amount
of impurities, between 0.5 and 2 %, which have a bed influence on the
mechanical and electrical properties of copper3. Amounts of 0.15 % of
phosphorous or 0.5 % of arsenic, dramatically decrease the electrical
conductivity of copper, as shown in Table 7.1.3 Hence, the improvement of
the electrical properties of copper is the main reason for electrorefining.
The second reason for electrorefining is the extraction of some noble
impurities, normally present in non-refined copper (anodic metal), such as:
gold, silver, platinum and palladium etc2.

As stated previously, anodic copper contains between 0.5 and 2 % of
admixtures that can be divided into four groups2:

while the impurities can react in the following manners2,3.
Metals of the first group, whose equilibrium potentials are more
negative (less noble) than copper (see Table 2.1) will during anode
dissolution be transferred together with copper into solution. Due to the
presence of sulfate ions in the electrolyte, lead ions will form lead sulfate
and fall into the anodic slime. Other metals, such as Ni, Co, Sn and Zn, will
be concentrated in the electrolyte, since their deposition potential is more
negative than the potential of the cathode, whose value is around 0.3 V. The
presence of these metals in the electrolyte has no bad influence, but it
necessitates periodical purification of the electrolyte. The presence of iron
ions is completely undesirable. Iron can be anodically dissolved in the form
of
but can not be deposited since its equilibrium potential is much more
negative than the potential of the cathode.
ions can be oxidized to
at the anode, but the value of the standard electrode potential of this reaction
(+0.77 V) is more positive than the potential of the anode
Bearing in mind the Nerst relation (Eq. 2.2) for the equilibrium electrode
potential:

it could be concluded that the potential is dependent on the ratio of the
and
concentration. If the concentration of
is much lower than that of
the equilibrium potential will become more negative than the actual
potential of the anode which could result in the oxidation of
to
This would decrease the concentration of
and increase the concentration
of
which leads to a possibility for the reduction of
to
at the
cathode. This unwanted closed circle would consume a part of the current
during the electrorefining process.

6. Electrowining

185

All the metals with which the electrolyte is enriched lead to a
consumption of free acid.
Insoluble impurities from the second group, such as
and
can not be anodically dissolved, but sediment and form anodic slime.
Only
can be dissolved, according to:

Hence, half of the copper is dissolved into the electrolyte in the form of
ions, while the other half forms a fine powder which partly reacts with
the air oxygen and sulfuric acid in the electrolyte, and partly sediments to
form anodic slime.
Metals of the third group as Au, Ag, Pt, Se and Te also sediment as the
anodic slime. They are more noble than copper and according the rule that
reactions with lower electrode potentials occur first (see section 2.3.1) at the
anode, these metals act as insoluble inert inclusions and fall down as the
anodic slime.
The most undesirable impurities are the metals of the fourth group: As,
Sb, and Bi. Their presence drastically decreases the quality of the cathode
copper deposit, as shown in Table 7.1. These metals are the most difficult to
be removed becouse of the values of their reduction potential which are near
the potential of copper, precisely between the equilibrium potential of copper
and hydrogen. However, their concentration in the electrolyte is usually very
small, so their deposition potential is more negative compared with the
potential of the cathode, and under normal conditions they are not deposited.
If for some reason the concentration of copper ions decreases, their
deposition could be expected. On the other hand, Sb and Bi could react with
As forming sparingly soluble arsenates, which floats in the form of fine foam
on the electrolyte and make electrolysis process difficult.4
The effects of electrorefining could best be illustrated by Table 7.2,2
which gives the distribution of impurities in the anodic slime, electrolyte and
cathode metal2.

186

7.2.1

Chapter 7

Technological scheme of copper electrorefining

The process of copper electrorefining is schematically illustrated in Fig
7.3.
Anodes of 99 – 99.5 % pure copper obtained, by pyrometallurgical
treatment, are prepared by casting after melting in the anode furnace. The
surface area of the anode depending on the capacity of the electrolytic cell, is
between 1 and
their width is between 40 and 50 mm, while their
weight is between 300 and 350 kg2,4. A part of the anodes is prepared to
cathodes by rolling into 1 mm thick copper sheets 5 kg in weight2,4. During
the manufacture of the anodes and cathodes, hooks for hanging the
electrodes onto electrode tracks are also produced.
After manufacture, the electrodes are placed into electrochemical cells,
the number of which can, depending on the capacity of the plant, be a few
tens connected in series or parallel.
Electrorefining of copper takes place in an electrolyte, a typical
composition of which is given in Table 7.33.

Thiourea and gelatine (10-100 g per 1 t of copper) are used as additives
for enhancing the quality of the deposit and preventing the formation of
dendrites which can produce short circuits during the electrolysis process3.
The conditions of the copper electrorefining process are2,4:
cathodic current density:
94 – 96 %
current efficiency
0.25–0.35V
cell voltage:
temperature of the electrolyte:
55–60°C
specific electrical energy consumption:

6. Electrowining

187

After the electrorefining process, cathodic copper of 99.97 â&#x20AC;&#x201C; 99.99 %
purity is dried, and further processed by melting, rolling into thin metal
sheets, wires or similar commercial products.

During electrolysis process the electrolyte is enriched in Ni, Sb, and As,
which are removed by regeneration, while the anodic slime is removed from
the electrochemical cell and transported for further processing. It is
important to mention that the process of the anode slime can bring in such
high incomes that the entire cost of electrorefining is covered.
As previously stated, copper electrorefming is an electrolysis process
with a soluble anode and theoretically the composition of the electrolyte
should not change during the process. However, in a real system some
composition changes do occur. First by the acid electrolyte dissolves some
amount of anodic copper in the presence of air oxygen, which results in an
increase in the concentration of copper ions, and a concurrent decrease in the
free acid concentration. Secondly, impurities from the anodic copper, which
did not sediment into the anodic slime, remain in the electrolyte and
additionally decrease the concentration of free acid.

188

Chapter 7

7.2.1.1
Refining of the electrolyte
In practice, two methods of correcting the electrolyte composition and
refining are in common use2.
The first way consists of removing copper, As and Sb by cathodic
deposition in separate electrochemical cells with insoluble anodes. This
procedure increases the concentration of the acid after which the solution is
concentrated by vaporization whereby crystallization of nickel, iron and
copper sulfates occurs.
The second procedure consists of the neutralization of the free sulfuric
acid by dissolving copper. Copper sulfate crystallizes from the saturated
solution. The rest of the electrolyte is freed from Cu, As and Sb by
electrolysis in a separate cell and from Ni, Zn and Fe by evaporation.
After this refining, the electrolyte, practically contains only sulfuric acid,
which is further, corrected by dissolving copper.
7.2.1.2 Processing of the anodic slime
The anodic slime is a precious crude from which noble and rare metals
and metalloids such as the platinum group metals, gold, silver, selenium,
tellurium etc are extracted1,2.
Copper is removed from the anodic slime by dissolution in hot dilute
sulfuric acid with air circulation, after which selenium and tellurium are
removed. Subsequently, by melting after the addition of quartz sand, sodium
carbonate and sodium nitrate, the anodic slime is transformed into an alloy
composed of 93 % silver, 3% gold, 1% copper, 0.05% palladium, 0.03%
platinum, and traces of other metals3.
Silver of 99.99 % purity is extracted from the alloy by the process of
anodic dissolution in a solution of silver nitrate. Owing to the high value of
the exchange current density which leads to the poor adhesion of the silver
deposit, some specific constructions of the refining cell are required which
enable the separation of the anodic slime collected in polypropylene bags
containing the anodes from the crystals of silver that drop from the cathode
to the bottom of the cell2.
After the process of silver electrorefining, the anodic slime is composed
of 95% gold, 5% silver and about 1% copper, palladium, platinum and some
other metals. After melting and casting, the anodes are subjected to a
electrorefination in the electrolyte containing
and free hydrochloric
acid. Owing to the high value of the electrode potential of gold deposition of
about 1.4 V, all of impurities, except silver which forms insoluble silver(I)
chloride, are dissolved and remain in the electrolyte2.
When the amount of platinum and palladium reaches a value of about
the electrolyte is replaced. These metals are subsequently recovered

6. Electrowining

189

in the form of ammonium salts:
and
By the
reduction of these salts, palladium and platinum are obtained2.
It is important to mention that the price of noble metals salts are a few
times higher than the price of the noble metals themselves.

Metal coating represents a metal electrodeposit which changes the surface
properties from those of the basic metal to those of electrodeposited one. It
should be adherent, nonporous and without internal stresses. A good adhesion
depends mainly on the preparation of the substrate for electrodeposition, resulting in a clean surface amenable to accepting a satisfactory deposit. Poor surface
preparation can cause peeling of the coating making it useless. Crashing and
peeling of a deposit can also be caused by internal stresses, which arise mainly
from the incorporation of foreign materials in the lattice. If lattice of an electrodeposited metal is free of such inclusions the mechanical properties of the
electrodeposit are practically the same as those of thermally prepared metal.
Finally, the surface properties of the basic metal can not be completely transformed into the properties of the electrodeposited one if the deposit is porous.1
The preparation of a substrate for electrodeposition is not connected with
metal electrodeposition and is not treated in this book. It is treated in details
elsewhere1,2. The appearance of stresses, which is partially connected with the
electrodeposition process, but has not yet been clarified is also not treated here.
On the other hand the effect of cementation on the adhesion of a coating
and the effect of deposition condition on the porosity of the metal deposits
can be treated in a semi-quantitative way.

8.1

CEMENTATION AND DEPOSITION FROM THE
COMPLEX SALT SOLUTIONS

If zinc is immersed in a copper sulfate solution the reaction

191

192

Chapter 8

occurs, whereby copper ions are converted to the metal, while the zinc
dissolves. Such deposits are spongy and dendritic in a number of cases, and
non-adherent as well. Hence, a good copper deposit on a zinc substrate can
not be formed in this manner. (It should be noted that under some
circumstances good deposits can be obtained by immersion deposition).1
In a cyanide-containing, bath the copper potential is sufficiently negative
so cementation does not occur and copper can be successfully deposited onto
zinc. This is due to the fact that the cyanide complexes of copper are very
strong so the potential of copper in such a solution is much more negative than
in simple salt solutions. On the other hand, the zinc cyanide complex is
relatively weak and the potentials of two metals become comparable so an
external power supply is required to deposit copper on the zinc from cyanide.
Analogously, from a copper sulfate solution, cementation of copper on
immersed iron occurs according to the reaction

but from a cyanide solution, copper can be successfully deposited using an
external power supply. In this case the complexes of both copper and iron
are very strong and cementation is theoretically possible if only the
reversible potentials only are taken into the consideration. The fact that
copper can be successfully deposited onto steal from cyanide containing
solutions is explained by the fact that the reaction between iron and cyanide
ions is very slow, and so the reversible potential is never reached.
It should be noted, concerning the electrodeposition from complex salt
solutions that the best coatings are obtained from cyanide solutions.
However, satisfactory deposits can also be obtained from some other
solutions, but many complex-containing baths produce unsatisfactory
deposits1. A list of suitable complex-containing baths and their working
conditions can be found in the literature3.
The quality of metal deposits obtained from complex salt solutions
depends on the deposition process parameters and of the adsorption of
anions onto the cathode. In the case of cyanide solutions, the deposition of
good deposits is ascribed to the decrease of the exchange current density and
increase of the value of the cathode Tafel slope due to the strong adsorption
of cyanide anions onto the cathode1,4.

8.2

THE POROSITY OF METAL
ELECTRODEPOSITS

The porosity is the most important property of good adherent and
stressless metal coating.

8. Optimum Conditions for Electroplating

193

The coverage of an electrode surface with deposited metal increases with
deposition time according to5:

where is the coverage, t is the time, k is a constant and j is the current
density of metal electrodepositon onto an inert substrate at a given
overpotential. Obviously, for systems with sufficiently low exchange current
densities the deposition current densities, on the inert substrate and on the
metal surface are the same (see section 3.1). The integral form of Eq. 8.2 is
given by:

Assuming that porosity,
surface, it can be written:

corresponds to the uncovered electrode

and

Equation 8.5 is confirmed by an experiment as illustrated in Fig. 8.1.

194

Chapter 8

The constant k in Eqs. 8.2, 8.3 and 8.5 can be evaluated in the following
way. At t = 0

and if one assumes a linear increases of
starts without overlapping, the equation

with time and that the covering

is valid. Obviously,

where
is the quantity of electricity which corresponds to one monolayer
of electrodeposited metal. Hence, k can be defined as the reciprocal of

Equation 8.3 can be now rewritten in the form:

where n

is the average number of electrodeposited monolayers.
Although Eq. 8.10 is only qualitative, it can be successfully applied for
discussing the dependence of the metal deposit porosity on the surface
coarseness. Let the local thickness distribution of an n-monolayer thick
(average thickness) deposit be:
fraction of surface
average thickness of deposit

1-2f

f

f

n

n+1

n-1

8. Optimum Conditions for Electroplating

195

The porosity of such a deposit will be given, according to Eq. 8.10, by:

Dividing Eq. 8.12 by Eq. 8.10 one obtains:

It is obvious that because

for f > 0, a smoother deposit will be less porous than a coarse one. This
means that the better the current density distribution on the microprofile and
the macroprofile, the lower is the porosity of a deposit.

8.3

THE CONDITION FOR THE DEPOSITION OF A
COATING WITH A MINIMUM POROSITY

The porosity of metal deposits depends on the thickness of the thin
surface film formation and the current density distribution on the
microprofile and the macroprofile.
Small and large values of the exchange current density and cathodic
Tafel slope enhance the formation of a surface film. This is realized by
deposition from complex salt solutions or in the presence of strongly
adsorbed additives. The best microprofile is obtained using deposition
current densities a little larger than the current density which corresponds to
the upper limit to the Tafel linearity in the presence of the some leveling and
brightening agents. Finally, the current density distribution is improved also
by increasing the Tafel slope of the deposition process and by decreasing the
ohmic resistance of solution.
Hence, the basic characteristics of plating baths which produce good
deposits should be:
a simple or complex salt solution from which the metal is deposited at a
sufficiently negative cathode potential relative to the substrate using a
deposition process characterized by a low exchange current density and a
large value of the cathodic Tafel slopes,
a conducting electrolyte which makes a low as possible ohmic
resistance of solution,

196

Chapter 8

the use of different kinds of additives, the synergetic effects of which
improve the deposition conditions in the way described in the previous
section.
There are also some other additives which are used occasionally for
different purposes or activators of anode dissolution.
The current regimes are also very important in electroplating processes.
The simplest current regime consist a short pulses of large current density
followed by prolonged deposition at a many times lower current density. In
this way nucleation takes place under more suitable conditions than exist
during deposition of a low current density, which allows less coarse deposits
to be obtained during prolonged electrodeposition. Regimes consisting of the
periodic repetition of different current or overpotential waves can also
improve the plating processes. It should be noted that the effect of
electrodeposition at a periodically changing rate in the presence of organic
additives is not completely understood yet.
In spite of the fundamentals of electroplating being very simple, (as
shown in Chapters 3 to 5) the overall process is complicated and consists of
a large number of sequences, the principles of which, as are given from the
theory to the industrial practice in Ref. 1-3, are required for complete
understanding of electroplating.

The aspects of the electrodeposition of individual metals are discussed in
other chapters. In this chapter, aspects of electrodeposition of alloys and
composite materials, electroforming as well as electrodeposition of metals
and alloys from nonaqueous solutions or room temperature molten salts are
discussed. In addition, anodic processes i.e. electropolishing, electromachining and electrochemical oxidation of metals is presented. These
processes are used in the production and developments of various materials,
which include electronics, automotive, aerospace, biomedical, corrosionprotection and energy conversion applications.

9.1

ELECTRODEPOSITION OF ALLOYS

Electrodeposited alloys have attracted significant attention, due to their
diverse applications in different industrial fields. Although different alloys
can be electrodeposited from molten salts or from organic solutions, in this
section only electrodeposition from aqueous solutions will be discussed.
Electrodeposition of over one hundred binary and ternary alloys has been
investigated, however, only several alloy-plating systems have attained
practical importance. Developments in the electronics, automotive and
aerospace industries have driven research in the field of electrodeposition of
alloys. Among these systems, alloys such as Fe-Ni, Ni-W, Ni-Mo, Pb-Sn,
Cu-Ni, Fe-Zn etc. should be mentioned.
Simultaneous reduction of two metal ions is possible when their
discharge potentials are equal, as presented by the equation:

197

198

Chapter 9

where
and
are standard electrode potentials of respective metals,
and are metal ions activities,
and
are cathodic overvoltages, and
are numbers of electrons, R is the gas constant, T is the absolute
temperature and F is the Faradayâ&#x20AC;&#x2122;s constant.
In the simple salt solutions, if the standard potentials of two metals are
close, and if overvoltages are negligible, changing the activities can bring the
discharge potential together. An example of this type is electrodeposition of
Sn-Pb alloys from fluoroborate solutions.
If the standard potentials are significantly different, changing activities of
metal ions cannot bring the discharge potential together. The most effective
way of bringing close together discharging potentials of two metals, which
are deposited simultaneously, is the formation of strong complexes with
metal ions. In this case, not only activities of metal ions, but also
mechanisms of deposition are changed. The complexing agents are chosen in
a way to reduce the activity of ions of more positive metal to a greater extent
then the activity of ions of less noble metal. It is important that the
overvoltage of more noble metal is higher than the overvoltage of less noble
metal. Among complexing agents, used in the electrodeposition of alloys,
cyanides, pyrophosphates, ammonia, fluorides, citrates, tartars etc. should be
mentioned. Sometimes, the addition of surface-active compounds to the
solution decreases the rate of reduction of more noble metals.
For the same composition, properties of electrodeposited alloys differ
from metallurgical (thermally) prepared alloys, which is a consequence of
differences in the crystallisation process. The electrodeposited alloys,
depending on the system, composition and electrolysis conditions may
represent true solid solutions, and as well they may contain different phases
consisting of various intermetallic compounds and of the mixture crystals of
pure components (eutectic-type of alloys).
The change in the phase structure for the alloys with the same composition is often observed with a change in the conditions of electrodeposition. It is obvious, then, that the properties, including composition of
electrodeposited alloys are determined by the electrodeposition conditions.
The main conditions determining properties of electrodeposited alloys are
classified in the following groups:
Composition of the plating solution, which includes concentration of
metals being deposited, concentration of complexing agents,
concentration of conducting electrolyte, pH, concentration of additives
etc,
Operating conditions, which include current density, temperature and
bath agitation, type of current (i.e. constant, pulsating, reversing etc.),

9. Electroplating and Surface Finishing

199

Other parameters, such as cell geometry, shape of the cathode, thickness
of the deposit, nature of the substrate, etc..
All of these parameters influence the composition and properties of
electrodeposited alloys in different ways, which usually depends on the
system being deposited. Therefore, a generalization of the effect of different
variables on the properties of electrodeposited alloys would be difficult,
unless impossible.
According to Brenner1, electrodeposited alloy systems are classified into
following five groups:
(i)
(ii)
(iii)
(iv)
(v)

The first three types, i.e. regular, irregular and equilibrium codepositions
are referred to as a normal codeposition. In the normal type of codeposition,
relative ratios of metals in the electrodeposited alloys are expected or
predictable on the basis of the equilibrium potentials of these metals in the
solution. While the equilibrium codeposition is a true normal codeposition,
in regular and irregular codepositions the more positive metals deposit
preferentially.
In the preferential deposition, the metal ratio in the deposited alloy is
greater than the ratio of metal ion concentration in the solution, as presented
by the equation:

where [A] and [B] are contents of metals A and B in the deposit, and
and
are metal ion concentrations in the solution.
In the regular codeposition, the alloy is deposited under diffusion control
conditions. In this case, the amount of more noble metal in the alloy
increases with an increase in the total metal ion concentration in the solution,
a decrease in the current density, a raise in temperature and with stirring. The
regular codeposition usually occurs when the potentials of the metals are far
apart, and when metals do not form solid solutions. Typical examples of
regular codeposition include Bi-Cu, Mn-Ni, Cd-Zn and Ag-Cu.
Irregular codeposition is related to systems in which deposition is not
under diffusion-control. Deposition in these systems is controlled by
irregularities of the potentials of metals in solution. This type of deposition

200

Chapter 9

occurs with metals which form solid solutions, and when potentials of metals
being deposited are close together. Examples of irregular codeposition are
Cd-Cu, Cu-Zn, Sn-Zn etc.
Equilibrium codeposition is characterized by the deposition, which is in
equilibrium with the both parent metals. This is the only type of deposition
in which the ratio of metal content in the deposit (plated at low current
density) is equal to their ratio in the solution. The equilibrium deposition is
rare and only a few plating systems, such as Cu-Bi and Pb-Sn
(electrodeposited from acidic solutions) were investigated. The alloys, which
do not have equilibrium with the both parent metals, belong to regular or
irregular plating systems.
Anomalous and induced codepositions are classified as abnormal
electrodeposition of alloys.
A very important factor that affects not only the composition, but also the
properties of electrodeposited alloys based on metals of the iron group is
simultaneous hydrogen evolution during electrodeposition. Most of the
hydrogen produced during electrodeposition forms molecular
which, as
bubbles, is removed from the cathode surface. Another small fraction of the
hydrogen becomes adsorbed in the crystal lattice of the electrodeposited
metals. The quantity of hydrogen included in deposits of ferromagnetic
alloys is approximately 0.45 at.%. According to Frumkin, hydrogen can be
incorporated in solid solutions or as hydride phases. 2 Thermal treatment of
these electrodeposits leads to removal of the incorporated hydrogen, causing
deformation of the crystal lattice and ultimately changing the properties of
the electrodeposited metal or alloy. In terms of electrocatalytic effects in the
evolution reaction changing the surface composition of an
electrodeposited alloy can lead to a time dependent current efficiency for
metal deposition relative to the
evolution rate.
The hydrogen evolution reaction on metals occurs in two main steps:

(i) discharge step

(ii) recombination-desorption step

or electrochemical desorption step

9. Electroplating and Surface Finishing

201

Sometimes, a third step involving sorption into the metal arises,
especially at transition metals

In the electrodeposition of alloys of the iron group of metals (Fe, Co and
Ni) from simple- or complex- salt solutions, the less noble metal is reduced
preferentially, and its relative content in the deposit is higher than that in the
bath. Such phenomenon is known as anomalous codeposition. Anomalous
codeposition is one of the most studied phenomena, especially on the Fe-Ni
alloys, due to their applications in the electronics industry as magnetic
materials. In the electrodeposition of Fe-Ni alloys the less noble metal, iron,
deposits preferentially and its relative content in the alloy deposit is higher
than that in the solution. The anomalous codeposition is observed in the
electrodeposition of all alloys based on the iron group of metals (Fe, Co, Ni).
Besides Fe-Ni, typical examples include Co-Ni, Fe-Zn, Ni-Zn etc.
There are elements in the periodic table, which cannot be deposited alone
from the aqueous solutions (e.g. Mo, W, P and Ge). These elements can
readily be deposited with the iron group of metals. The phenomenon is known
as induced codeposition. In the induced codeposition, the iron group of metals
is referred as inducing metals, while Mo, W and P are reluctant elements.
One of the more widely studied examples of anomalous codeposition is
electrodeposition of Ni-Fe alloys. Processes occurring during
electrodeposition of Ni-Fe alloys can be summarized in terms of reactions:

and

with

or

The experimental studies of electrodeposition of Ni-Fe alloys have
shown that:

202

Chapter 9

(i)
(ii)

The ratio of Fe to Ni is higher in the alloy than in the electrolyte, and
The presence of Fe(II) in the solution inhibits the discharge of Ni.

Several models appeared in the literature with an attempt to explain the
anomalous nature of electrodeposition of Ni-Fe alloys. The early models did
not receive significant attention. Dahms and Croll postulated one of the most
cited models for the electrodeposition of Ni-Fe alloys in 1965. 3 This model
is based on the assumption that due to simultaneous evolution of hydrogen
during electrodeposition an increase in pH near the electrode surface occurs.
This increase in pH, as the authors assumed, causes the precipitation and
adsorption of hydroxides at the electrode surface. The inhibition of discharge
of Ni(II) ions was considered as a consequence of a blockage of Ni(II) ions
by Fe(II) hydroxide, which is formed to a greater extent than Ni(II)
hydroxide. In this way, an increase in the amount of iron content was
attributed to the adsorption and incorporation of
into the alloy
deposit. According to the Dahms and Croll hypothesis, the inhibition of
nickel discharge occurs at potentials where hydrogen reduction exceeds its
mass transport limit.
Several modifications of the hydroxide-based model appeared in the
literature. These models are often in agreement with experimental data,
however, their validity is determined by their ability to simulate the
measurable macroscopic aspects of deposition.
Although simultaneous hydrogen evolution is an important factor
influencing properties of electrodeposited alloys, according to later
experimental studies it appears unlikely that this reaction causes a significant
increase in pH near the cathode surface. On the other hand, the anomalous
codeposition occurs even at low current densities, where the hydrogen
evolution reaction is negligible4, and therefore cannot cause a high increase
in pH which may lead to the precipitation of Fe(II)-hydroxides.
To explain the anomalous codeposition, Matlosz proposed a two-step
reaction mechanism in which electrochemical (rather than chemical) kinetics
is emphasized.5 By a combination of a two-step reaction mechanism for the
electrochemical reduction of the single metals depositing alone, a
competitive adsorption model was developed. This mechanism is described
by the following reactions:

where the symbol M denotes either Fe or Ni.

9. Electroplating and Surface Finishing

203

The experimental results showed that nickel deposits normally at low
overpotential. Deposition of nickel is suddenly inhibited at a specific
cathodic polarization and, the codeposition process does not influence
deposition of iron. This behavior is attributed to a higher surface coverage of
the adsorption sites on the electrode by the iron intermediate species, which
is in a good agreement with the proposed model. The mechanism suggests
that the pH near the cathode surface plays only a secondary role and is not
the necessary condition for an occurrence of the anomalous codeposition.
The hydroxide concentration, at the electrode surface does not change the
mechanism. The electrodeposition of iron-reach alloys at higher potentials
could be avoided with a decrease in the Fe(II) concentration in the solution.
Based on the theoretical calculations and experimental observations Vaes
et al. concluded that the metal hydrolysis does not play a determining role in
anomalous codeposition of Ni-Fe alloys.6
The composition of Ni-Fe alloys depends on plating conditions including
the composition of the electrolyte. Generalization of these dependencies is
difficult due to different experimental conditions, and the various
electrolytes used in production of these alloys.
Based on the experimental evidence that the current efficiency for nickel
deposition is practically constant a simple equation for the dependence of
alloy composition on the parameters of periodically changing current is
derived.7 This equation is presented as follows:

where %Fe is the percent of iron in the deposit, CE(Ni) is the current
efficiency of nickel deposition,
is the cathodic charge, Q is the total
charge and k is a constant. The function
depends on parameters of
periodically changing current. As this equation shows, there is a linear
relationship between the composition of Ni-Fe alloys and Qc/Q function.
The experimental results confirm the validity of the equation (9.13), as
presented in Figure 9.1.
Similarly, for the electrodeposition of Ni-Fe alloys under the
potentiostatic conditions, the following equation is applicable:

where k and kâ&#x20AC;&#x2122; are constants and E is depositing potential. For a certain
range of potential, a linear relationship between log (%Ni/%Fe) and E is
observed as it is confirmed by the experimental results (Figure 9.2).

204

Chapter 9

One of the most studied systems of induced codeposition is electrodeposition of Ni-Mo alloys because of their corrosion and wear- resistance
properties as well as for their electrocatalytic effect on the hydrogen
evolution reaction in alkaline solutions. Ni-Mo alloys are electrodeposited
from citrate or pyrophosphate solutions. The composition of Ni-Mo alloys
depends on concentration and mass transport of Ni (II) and molybdate
species. 8 If the relative concentration of Ni(II) in the solution is significantly
higher than the relative concentration of molybdate ions, the molybdenum
content decreases with current density, but increases with the rotation rate.
Contrary, if the concentration of molybdate ions is larger than the
concentration of Ni(II), an increase in the current density leads to an increase
in molybdenum content in the alloy. However, the alloy composition for this
case is independent on the rotation rate. This suggests that that the mass
transport does not influence the composition of Ni-Mo alloy when the
concentration of molybdate is significantly larger than the concentration of
Ni(II) in the plating solution.

9. Electroplating and Surface Finishing

205

The researchers proposed several mechanisms for the electrodeposition
of Ni-Mo alloys. It seems that the most feasible is the mechanism proposed
by Chassaing et al. 9. They reported a two step mechanism for the
electrodeposition of Ni-Mo alloys, which can be presented as follows:

(i)

Formation of
oxide

which, in the presence of Ni(II) gives a mixed

and

(ii)

Reduction of
hydrogen, thus

by hydrogen to

with included

206

Chapter 9

The simultaneous hydrogen evolution during electrodeposition of Ni-Mo
alloys strongly influences the properties of these deposits. The
electrodeposited Ni-Mo alloys show formation of a “cauliflower” surface
morphology, high surface area, formation of voids, pits and cracks (Figure
9.3) and a gradient in composition. However, under certain conditions
electrodeposition of crack-free, uniform deposits is also possible.

Electrodeposited alloys based on the iron group of metals are usually
crystalline. Codeposition of P and B with the iron group of metals is especially
important because the incorporation of these elements into a deposit influences
the structure of the electrodeposited alloys. The incorporation of phosphorus in
the alloy deposit is essential factor leading to the production of amorphous or
nanocrystalline alloys. 10 Comparison of XRD patterns of “as
electrodeposited” Ni-P, Co-P and Ni-Co-P alloys is presented in Figure 9.4.
The broad peaks associated with these deposits correspond to an amorphous
structure. This result confirms that the alloys based on iron group of metals
containing more than 8 % of phosphorus, particularly Ni-P alloys are
amorphous. When heat treated at temperatures above 350 °C, these alloys
completely devitrify, forming mixtures such as Ni,
Co,
etc.,
depending on the overall alloy composition (see Figure 9.5).
Among other alloy plating systems that have attracted the attention of
researchers, the multilayer films or compositionally modulated multilayers
should be mentioned. 11-13 Compared with pure metals, the compositionally
modulated multilayers with distinct sublayers have unusual and enhanced
mechanical, electrical, optical and magnetic properties. Examples of these
systems include Cu-Ni, Cr-Ni, Cu-Co, Ag-Pd etc.

9. Electroplating and Surface Finishing

207

Two different ways, known as single bath technique (SBT) and double
bath technique (DBT) have been proposed for deposition of compositionally
modulated multilayers. In the single bath technique, deposition is carried out
from one bath (plating solution) which contains ions of both constituents of
the multilayer film. This is achieved by periodical variation of the applied
potential or current between a value corresponding to deposition of one
metal to a value corresponding to predominant deposition of the other
component. The thickness of the layer can be modified in a wide range,
depending on the time frame and potential or current variation. For the
production of multilayeral coatings deposition under periodically changing
current conditions is well suited.13
The double bath technique involves the use of two different plating
solutions containing ions of individual constituents of the multilayer film. In
this type of deposition, the substrate is successively moved from one to the
other bath and layers of pure metal are deposited in each of the individual
baths. The double bath technique is often accompanied by dissolution of
plated metal, displacement reaction and passivation of the surface, due to
removal from one to another bath. Between the two plating steps, substrates

208

Chapter 9

should be rinsed to avoid contamination of plating solutions, which increases
the wastewater generation.

In the single bath technique the minimum layer thickness can be as low
as 1 nm, while for the multilayers produced by means of double bath
technique the minimum film thickness is estimated at about 25 nm. The
individual metal sublayers usually grow epitaxially on top of one another.
Based on the mixed potential theory, in which the measured current in an
electrochemical system is equal to the sum of anodic and cathodic partial
current, i.e.,

where I is the measured current,
is the partial anodic current and
is the
14
partial cathodic current, Landolt proposed that the composition of
electrodeposited alloy differs from that of the electrolyte and depends on

9. Electroplating and Surface Finishing

209

both kinetic and thermodynamic quantities. According to the Tafel equation,
for the cathodic partial current of species i,

where
is the exchange current density,
is the inverse of the cathodic
Tafel coefficient
is the overvoltage given by
(E is the
applied potential and
is the equilibrium potential of species i), the ratio
of partial current densities for two species A and B is given by the equation:

where

is defined by:

The equation (9.20) shows that the composition of an alloy does not
depend on potential only if

On the other hand this equation demonstrates that the composition
depends on the exchange current density through the ratio
Based on
the mixed potential theory and the above equations, Landolt classified
electrodeposition of alloys into the following groups:
(i)
(ii)
(iii)

In the non-interactive codeposition, the partial currents are independent of
each other. A typical example of this type of codeposition is the
electrodeposition of nickel-copper alloys. 15
The charge-transfer coupled is a type in which the partial currents depend
of each other. This type of codeposition is divided further into two
subgroups designated as inhibited codeposition and catalyzed codeposition.
The examples of the inhibited codeposition include electrodeposition of
zinc-nickel or iron-nickel alloys. This type is quite similar to the anomalous
deposition as described by Brenner. The partial current density for

210

Chapter 9

deposition of more noble metal, e.g., nickel, during the electrodeposition of
alloy is much lower than the current density, when this metal is plated alone.
On the other hand, the partial current for deposition of less noble metal, e.g.,
zinc, is not affected by the presence of nickel.
The catalyzed deposition (Landolt’s classification) is exactly the same as
the induced deposition (Brenner’s classification). Examples of this
codeposition are electrodeposition of Ni-Mo and Ni-W or Ni-P alloys.
Finally, in the mass transport coupled codeposition the partial current of
the component A depends on the transport of component B. This type of
codeposition includes systems in which the simultaneous hydrogen evolution
occurs during electrodeposition of alloys (e.g. electrodeposition of Fe-Ni or
Zn-Ni alloys). Under conditions of simultaneous hydrogen evolution due to
consumption of protons, a local increase in pH depends on mass transport
conditions and the buffering capacity of the electrolyte, and may effect the
mechanism and kinetics of electrodeposition of alloys.
The advantage of Landolt’s classification over Brenner’s is in the fact
that, the former takes into consideration not only thermodynamics, but also
charge transfer kinetics and mass transport. However, some systems such as
for example electrodeposition of Fe-Ni alloys or Zn-Ni alloys as per
Landolt’s classification can belong to either charge-transfer coupled or
transport coupled codeposition.
Although many systems of electrodeposition of alloys have similarities
there are significant differences. These differences arise as a consequence of
different conditions of electrodeposition, which includes solution
composition and operating conditions. It is obvious that more research is
required to evaluate the significance of different parameters for a full
explanation of features of alloy electrodeposition.

9.2.

ELECTRODEPOSITION OF COMPOSITE
MATERIALS

Composites produced by electrodeposition include materials with metallic,
oxide or polymer matrices, in which solid particles or fibres are codeposited
and uniformly distributed in the deposit. 16 The inert particles used in the
electrodeposition of composite materials are usually 0.01 to
in diameter
and are selected from alumina, boron, carbon, silicon carbide, titanium
dioxide, tungsten etc., depending on applications. The particles are uniformly
dispersed in the plating solution by a mechanical or ultrasonic agitation.
During electrodeposition, the inert particles become positively charged and as
such, attracted by the cathode and incorporated into the electrodeposited metal,
alloy, oxide or polymer. Electrodeposition of composite materials with
metallic matrices is usually carried out in order to improve their mechanical

9. Electroplating and Surface Finishing

211

and tribological properties, although other characteristics such as corrosion
and thermal resistance can also be significantly advanced.
The metal matrices may include nickel, cobalt, copper, zinc, precious
metals and related alloys. Solid (inert) particles, which are incorporated into
metal deposit, include oxides (
etc.), carbides (SiC,
), graphite, diamond or boron nitride particles, polymer powders
(polytetrafluorethylene, polyvinyl chloride) and other components such as
salts
or some pigments. The incorporation of submicron
powders such as
and
in the nickel-based
metallic matrices significantly increases the corrosion resistance. Particulates
like WC, diamond and SiC protect metal from abrasion, while
and
polytetrafluorethylene (PTFE) and graphite reduce the friction coefficient of
the composite materials.
For deposition of composite materials with metallic matrices, solutions
similar to those in the electrodeposition of metals and alloys are used. The
main difference is that solutions used in the electrodeposition of composite
materials contain dispersed fine particles or fibres. Codeposition of inert
particles into metal matrix is influenced by the adsorption of particles on the
cathode. The content of particles in the deposit is influenced by their
concentration in solution, additives, pH and current density. Most of the
studies show that the content of deposited particles in the metal matrix
increases with increasing particle concentration in the solution. In terms of
particle size, different results have been reported for the same systems.
Additives, such as brightners or wetting agents, influence codeposition of
inert particles. Quite opposite observations on the effect of brightners or
wetting agents on the amount of solid particles occluded into deposit were
reported. In some cases, in the presence of these substances, an increase in
the amount of particles is observed. Other researchers, in contrast reported a
decrease in the particle content with an addition of wetting agents.
Current density significantly influences the codeposition of particles.
Although, some researchers reported no influence of current density on the
amount of particles deposited, most commonly, the observed dependence of
current density on the particle concentration passes through one or several
maximums. Typical examples are presented in Figures 9.6 and 9.7. As
shown in Figure 9.6, the dependence of the amount of
in
deposit on current density, passes through two maximums for different
rotation speeds. 17
Similar dependencies of the amount of SiC in the Co-SiC deposit on
current density, for different concentrations of SiC in the solution are
presented in Figure 9.7. 18 A presence of particles in the plating solution
increases the current density for the same cathodic potential. This indicates
that the presence of particles in the solution causes a depolarization of the
cathode.

212

Chapter 9

The effect of pH on codeposition of solid particles such as
or SiC
into nickel matrix was investigated. The results have shown that an increase
in the amount of particles in the deposit is observed with an increase in pH
up to 2. With a further in crease in pH, no change in the amount of
codeposited particles is observed. 16 Similar results have been reported for
other composite materials systems, where electrodeposition is carried out
from acidic solutions.
In order to produce coatings with a homogenous composition the solid
particles should easily be transported through the solutions and their
precipitation on the bottom of the plating cell should be avoided. This is
usually achieved with a good agitation and with the addition of
corresponding surfactants.
Generally agitation of the solution enhances the particle transport and
increases their amount in the deposit. However, a too high agitation causes a
decrease in the amount of codeposited particles. Consequently, the
dependence of the rate of the solution agitation on the concentration of the
particles shows a maximum. The stability of the suspension determines the
quality of deposited composite material, and depends on the rate of
sedimentation of solid particles, The rate of sedimentation, for an ideal case

9. Electroplating and Surface Finishing

213

of spherical particles, is described by the Stokes law with the following
formula:

The equation (9.23) shows that the rate of sedimentation, v, is directly
proportional to the particle size, i.e. their diameter,
particles density
density of the electrolyte
and indirectly proportional to the viscosity of
the solution,
In order to avoid the agglomeration and precipitation of particles in the
electrolyte, addition of surfactants such as tannin, gelatine etc. is
recommended. The addition of surfactants increases the wettability of
particles, and stability of the suspension. Surfactants used in the
electrodeposition of composite materials are classified as:

214
(i)
(ii)
(iii)

Chapter 9
cationic,
anionic and
nonionic.

The cationic surfactants confer a net positive charge of the particles,
which attracts them electrostatically to the cathode. Contrarily, the anionic
surfactants (such as some brightners or wetting agents) confer a negative on
the particles, which leads to a decrease in the amount of codepositedparticles. The use of non-ionic surfactants usually promotes codeposition of
solid particles. The surfactants, which are used in the electrodeposition of
composite materials, can to a certain extent, deteriorate the quality of the
coating (high internal stress and brittleness). This occurs due to adsorption of
these substances on the electrode surface.
Many authors investigated the mechanistic aspects of electrodeposition of
composite materials. The early models suggested that the particles with a
positive surface charge are drawn by electrophoresis, or due to agitation,
transported to the cathode and mechanically entrapped by the growing metal
layer. The idea of mechanical entrapment was rejected, and the attraction of
solid particles by the cathode is attributed to the electrostatic force.
One of the most cited models for electrodeposition of composite
materials was developed in 1972 by Guglielmi.19 The model is based on two
successive steps, and considers both electrophoresis and adsorption
phenomena. According to this model, solid particles are surrounded with a
cloud of adsorbed ions. In the first step, when particles approach the cathode
they become loosely adsorbed at the surface. The second step involves a
strong, irreversible adsorption of these particles at the cathode and their
incorporation into the growing metal layer. The strong adsorption of solid
particles is preceded by a loosing of their ionic cloud. This model takes into
consideration most experimental parameters, however it does not explain the
appearance of the maximum in the particle content versus current density
curves. Most importantly, this model neglects the mass transfer.
The model proposed by Guglielmi was extensively used by other
researchers as a basis for development of other mechanisms, for the
electrodeposition of composite materials. As a consequence, several
mechanistic models appeared in the literature.l7,18,20 These mechanisms were
proposed in an attempt to explain the characteristics of electrodeposition of
composite materials, however, further studies are required for a more general
understanding of this process.
The generally accepted mechanism for electrodeposition of composite
materials involves the transport of particles from a solution to the electrode
surface by agitation and their incorporation in the metal matrix by reduction
of adsorbed ions. The literature survey shows that the particle concentration

9. Electroplating and Surface Finishing

215

in the electrolyte, agitation and metal growth mechanism play important
roles in the electrodeposition of composite materials.
The electrochemical impedance spectroscopy study of codeposition of
SiC and
particles with nickel suggests that particles suspended in a
plating bath increase the roughness of the electrode surface. 21 Particles
adsorbed, but not embedded in the electrode, remain separated from the
electrode surface by a liquid film, which is thicker than the width of the
electrical double layer. This liquid film, according to the authors is the key
factor controlling the electrolytic deposition of particles.
While the early work was restricted to electrodeposition of composite
materials with metallic matrices, the field has recently been extended to the
development of other matrix materials (i.e., polymers or ceramics). 22 In order
to be used as a matrix, the material must fulfil the following requirements:
(i)
(ii)

must be depositable either on the cathode or on the anode, and
must be electronically conductive.

Polymeric matrix materials, used in the electrodeposition of composite
materials include polypyrrole, polyaniline and polythiophene. These
materials were deposited from aqueous or aprotic solutions. The dispersed
particles are usually Pt, Pd,
etc.. Composite materials
with polymeric matrices were mainly investigated as electrode materials in
the fuel cell technology for either oxygen reduction or hydrogen oxidation.
23,24
Metal particles, incorporated in polymer films, act as catalytic sites for
the electron-transfer processes. Typical examples include platinum
nanoparticles incorporated into polypyrrole, or, palladium aggregated into
the polyaniline. In the electrodeposition of composites with polymer
matrices two main routes are followed. The first, similarly to
electrodeposition of metal- or oxide- matrix composite, is based on the
electrolysis of suspensions of the dispersed phase in solutions of monomer,
which is converted to a solid phase (polymer) by electropolymerization.25,26
In the second route the electropolymerization is first occurred and then
formation of metal clusters within the polymer. 27,28 An example of this type
of composite material is polypyrrole film containing highly dispersed
platinum particles.
ions are reduced to Pt째 particles with an average
size of about 10 nm, according to the reaction:

where PPy denotes polypyrrole.
Oxide matrices in the electrodeposited composite materials include, but are
not restricted on
and non-stoichiometric W(VI, V) oxides. 29-33

216

Chapter 9

Composite materials with oxide matrices are usually investigated for the
electrocatalysis purposes.
and
matrices are selected, since they
have a high electronic conductivity and they are anodicaly depositable from
or
solutions. Electrodeposition of composites such as
and
is investigated for the applications as anodic
materials for the oxygen evolution reaction. Codeposition is carried out from
or
electrolytes containing
suspended particles (less
than
The incorporation of
in
or
matrices leads to an
increase in the surface roughness and effective electrode area, which is
favorable for the oxygen evolution reaction.
A composite material containing non-stoichiometric W(VI,V) oxides as a
matrix and Pt microparticles by the cyclic voltammetry, during the reduction
cycle is recommended for the reduction of molecular oxygen in the fuel cells
applications. 33
The features of electrodeposition of composite materials with oxide
matrices are similar to electrodeposition of composites with metallic matrices.

9.3

ELECTROFORMING

Traditional electroforming is a method of producing metallic components
by electrodeposition over a mandrel or mold, which can subsequently be
separated from the deposit. The separated metallic component produced by
the electrodeposition, represents itself a finished part. Consequently, in the
traditional electroforming process, the surface of a mandrel is prepared in
such a way that plated metal does not strongly adhere to the substrate. 34,35
Thus, the metal is readily separated from the substrate after plating.
However, the adhesion has to be sufficient in order to avoid the separation of
deposited metal before the electroforming process is completed.
For larger articles, electroforming is used in automotive, aerospace,
biomedical, jewellery and musical industry applications. The applications of
electroformed parts are found in the continuous copper foil used in the printed
circuit industry, nickel screen or mesh patterns for printing in the textile
industry, mold stampers for the compact audio and video disks, seamless
cylinders used in copying machines, components in rocket motors, etc..
Depending on the design of the electroform, and/or, the quantity of parts
required, mandrels may be either permanent or disposable. The permanent
mandrels are used repeatedly, while disposable mandrels are destroyed after
removal from electroform. Permanent mandrels are preferred when the
electroform has no undercut surfaces and can easily be separated without
damage. In these cases, the mandrel is dissolved or melted to free the
electroform.

9. Electroplating and Surface Finishing

217

The material used for mandrel must be dimensionally stable, since its
surface morphology is reproduced exactly down to submicroscopic level,
with nearly atomic resolution on the electroform. This duplication of the
surface details accounts for many applications of electroforming process.
Typical materials used as permanent or disposable mandrels are listed in
Table 9.1. It should be noted that every single material used as a mandrel has
certain advantages or disadvantages. 34 Consequently, certain precautions
such as machinability, scratching, corrosion resistance etc. should be taken
into consideration in order to achieve the successful operations of the
electroforming process.
In the early approaches, for some applications where formulations based
on waxes were used as mandrels, in order to make the surface of these
materials conductive, the graphitization was frequently applied.

When electroforming is performed on dielectric surfaces, they are usually
coated with thin Ag film that has a limited adhesion. For this purpose , either
vacuum or electroless deposition techniques can be used. Other metals such
as Ni, Cu, Cr, etc., can be applied using available techniques (i.e. vacuum
deposition or electroless plating) as long as they provide a limited adhesion
and permit a relatively easy separation of the electroformed part from the
mandrel. After a proper degreasing and cleaning, the stainless steel or copper
mandrels are usually overcoated with a thin flash of chromium, so that the
electroformed part is easily removed from them. Copper, nickel and iron are
generally used for electroforming purposes. Hard chromium plating is
applied on electroforms when wear resistant surfaces are required. In order
to obtain a desirable quality of electroforms, electrolytes used in the process
should be free of contaminants. Removal of contaminants is realized by
continuous filtration through activated carbon.
The application of electroforming in the electronics industries is a very
important approach leading in the production of various parts, such as thin
film heads, thin film chip carriers, integrated magnetic minimotor, etc.. These
parts would be difficult or impossible to make by other methods. In the
electronic applications, the electroforming is known as plating through
lithographic masks, pattern plating, additive plating etc. In contrast with the
traditional electroforming, the metal plated through a resist mask does not
represent a finished product by itself. This plated metal is rarely removed from
the substrate. It remains on the substrate and is an integral part of the

218

Chapter 9

electronic or magnetic device. Electronic or magnetic devices often contain
several electroformed layers produced via plating through a mask. These
layers may have different patterns and they are usually separated by dielectric
substrates. In this way, in contrast with the traditional electroforming metals
plated through a mask must have a very good adhesion with the substrate.
Excellent adhesion is achieved by evaporation or sputtering of refractory
metals such as Cr, Ti, Ta etc., directly onto dielectric substrate. This layer of
refractory metal provides a bridge between the dielectric and layer then
overcoated with Cu, Ag, Au or Ni in order to form conductive layer
(cathode) for electroplating.
Plating through mask technology is schematically presented in Figure
9.8. The dielectric substrate is metallized with the refractory metal (Cr, Ti,
W etc.) to provide an adhesion layer with a thickness of 5 to 50 nm.

9. Electroplating and Surface Finishing

219

The adhesion layer is overcoated with a conductive metal such as Cu, Ni,
Au etc., which usually depends on the applications. In the next step, the
surface is coated with an organic polymer, and then through a mask exposed to
light, x-ray or e-beam in order to form-a pattern. Areas exposed to the light
will be depolymerized. Consequently, these areas are dissolved in the
developing solution, and after rinsing, drying and removal of residual organic
impurities, parts are plated through holes to achieve the desired pattern. After
the plating process is completed, the residual resist is removed by a second
exposure to the light, x-ray or e-beam and dissolution in the developing
solution. The adhesive and conductive layers are removed by chemical
etching. By using the e-beam and x-ray lithography instead of optical
lithography, pattern dimensions as small as
have been produced.36,37
Although electroplating through mask technology has applications in the
production of many devices for computers and other products, it still needs
future development.

9.4

ELECTROPLATING FROM NON-AQUEOUS
ELECTROLYTES

Electroplating of metals from non-aqueous systems is often referred to as
plating from water-free inorganic and organic solutions and does not include
deposition from molten salts. It is usually carried out at or below 100 Â°C.
Electroplating from non-aqueous electrolytes is particularly important for
metals, which cannot be deposited from aqueous solutions.38
If the reduction of metal ions takes place at potentials more negative than potential of discharge of water, the main cathodic process is the hydrogen evolution reaction. In this case, metals with sufficiently negative standard potentials
cannot be deposited from aqueous solutions. Due to hydrogen evolution, the
alkalinity near the cathode increases, leading in this way to the precipitation of
metal hydroxides or the deposition of oxides at the electrode surface.
The analogous processes may occur in organic protic solvents, as a consequence of dissociation and formation of
ions. Protic solvents contain
hydrogen that is attached to oxygen or nitrogen and hence is appreciably acidic. In order to avoid hydrogen evolution reaction, aprotic solvents are recommended. These are polar solvents of moderately high dielectric constants,
which do not contain acidic hydrogen. They dissolve both organic and
inorganic reagents, but in dissolving ionic compounds solvate cations most
strongly, and leave the anions relatively encumbered and highly reactive.
Aprotic organic solvents have a relatively high electrochemical stability, since
their reduction takes place above â&#x20AC;&#x201C;3.0 V, and they can be anodically oxidized
at 1.0 V to 1.5 V. The electrode material determines the region of the electrochemical stability of these solvents.

220

Chapter 9

Metal ion sources for the electroplating from non-aqueous solutions are
selected from suitable inorganic or organic compounds with a good
solubility and a high conductivity. The nature of the dissolved salt and
structure of cations and anions of non-aqueous electrolytes influences more
significantly the electrocrystallization, than is the case for aqueous solutions.
This effect is attributed to an increased complexation and specific action
between dissolved compound and solvents. In this way the nature of the
organic solvent and electrolyte determine the possibility of metal deposition.
Advantages of non-aqueous electrolytes for plating purposes include a
larger voltage window of solvent stability, very low or no reactivity with
substrates, formation of variety of complex ions in solutions and dissolved
salts do not hydrolyze. 39 A larger window allows a greater flexibility in
selecting cell-operating voltages. No reactivity of non-aqueous electrolytes
with substrate makes possible to plate metals such as for example uranium
with nickel or zinc38, which react with aqueous types of electrolytes.
Disadvantages of non-aqueous electrolytes are associated with toxicity,
flammability, explosion, low electrical conductivity, sensitivity to water and
a relatively high cost. Electrodeposition of metals from organic solutions
requires specially designed systems, which must be protected from oxygen,
carbon dioxide and moisture.
In terms of solvents, non-aqueous electrolytes, used in electrodeposition
of metals and alloys may be divided into two large groups: organic-solvent
based and inorganic-solvent based. Examples of organic solvents are
benzene, toluene, ethyl pyridinium bromide, diethyl ether, ethyl benzene,
tetrahydrofuran, etc.. The number of inorganic solvents used for plating
purposes is significantly smaller. The inorganic solvents include liquid
ammonia, thyonil chloride and sulfuryl chloride.
Metals depositable from non-aqueous systems can be divided into two
large groups. 38 In the first group are listed metals, which cannot be
deposited from aqueous solutions, i.e. metals of the first group of the
periodic table (Li, Na, K), metals of the second group (Be, Mg, Ca), metals
of the third group (Al) and metals of the fourth group (Ge, Ti, Zr). To this
group are also added metals of the fifth and the sixth groups of the periodic
table (i.e. V, Nb, Mo and W). Note that metals such as Mo and W are listed
into the first group, although they can be deposited from the aqueous
solutions, but not in the pure state. Metals such as Mo and W are readily
deposited from aqueous solutions, however only in the presence of iron
group of metals (i.e. Ni, Co, etc., see the section related to the electrodeposition of alloys).
In the second group of metals, which can be plated from non-aqueous
solutions are listed metals usually deposited from aqueous systems (i.e., Cu,
Zn, Co, Sn, Ni etc.). Although these metals are commonly deposited from

9. Electroplating and Surface Finishing

221

aqueous solutions, for some specific requirements they can also be deposited
from non-aqueous solutions.
Very little or no success is achieved in deposition of metals of the first
group in their pure state from non-aqueous solutions. This may be due to
their limited industrial applications. Metals that are not deposited so far from
non-aqueous electrolytes in their pure state include calcium, strontium,
barium, the lanthanides and actinides, titanium, zirconium, hafnium,
vanadium, niobium, molybdenum, tungsten and tantalum. However,
literature shows that these metals were deposited from non-aqueous
solutions as alloys, although at low current efficiency. 40
The most important metal from the first group, in terms of platability
from non-aqueous solutions, is aluminum. 41-43. Deposition of aluminum
from non-aqueous solutions has attracted researchers and industry for the
two simple reasons. First, it cannot be deposited from aqueous solutions and
second it has immense applications in various technical fields. Electroplating
of aluminum is carried out industrially, although to a limited extent due to
technical difficulties, and a relatively high cost of operation. 44 This bath
consists of aluminum alkyl and sodium or potassium fluoride dissolved in
toluene and a high purity aluminum used as the anode. The cell operates at
100 째C with 100 % of anodic and cathodic current efficiencies. The bath has
an excellent throwing power.
Aluminum is electroplated in an enclosed plating cell to prevent reactions
with oxygen, carbon dioxide or water from air, which would degrade the
electrolyte and shorten its useful life. However, with the special process
control and plating equipment, the electrolyte is stable, and is not consumed
during the plating for one year.
Different types of electrolytes that may be used in the electroplating of
aluminum from non-aqueous solutions are listed in Table 9.4.1. Practically,
all these electrolytes with exception of aromatic solvents work under
extremely dry conditions (no presence of water). All these solutions are
unstable up to a certain degree, which is disadvantageous in their industrial
applications. On the other hand specific precautions should be taken since
some of these solutions are very flammable. While the aluminum is plated
on the cathode, the main anodic process is dissolution. In this process
aluminum anodes are used.
In the alkyl benzene electrolytes the cathodic current efficiency is
estimated at about 50 to 80 %, while the anodic efficiency is close to a 100
%. Due to anodic dissolution of aluminum, an increase in the aluminum-ion
concentration is observed. The excess of aluminum ions reacts with bromide
ions, which are introduced in the solution with an addition of HBr.
Electrodeposition of aluminum in alkyl benzene electrolytes is described
by the reaction:

222

Chapter 9

A positive influence of water is attributed to a formation of dischargable
hydroxo-complexes, according to the reaction:

Discharge of these hydroxo-complexes is then given by the equation:

In the real systems, however, the discharge mechanism is more
complicated and involves side reactions such as evolution of
HBr
formation and a possible polymeraization of the solvent and impurities.
Electrodeposited aluminum is of a higher purity (99.5 to 99.999 %) than the
anode, since the impurities and alloying elements are insoluble in the
electrolyte. Impurities are continuously eliminated by filtration of the
electrolyte. The pure aluminum layer becomes pore-free at a thickness
greater than
The thickness of electrodeposited aluminum can reach
if some special applications are required. The aluminum coating has
the very good ductility, corrosion resistance, and when electropolished it is
convenient for the production of mirrors of high optical quality. 45
As mentioned above, the second group includes metals, which can be
electrodeposited from aqueous solutions (i.e., Cu, Zn, Ni, Co, Ag, Au etc..).
The electroplating of these metals from non-aqueous solutions at the present
does not have significant industrial applications, and therefore is mostly of
academic value. The research shows that electrodeposition of this group of
metals takes place in polar types of solvents (solvents that have active
functional groups such as
CO,
and
in which, the same
type of salts used in the aqueous solutions are dissolved (providing they have
sufficient solubility). As examples of studied systems, electrodeposition of
various metals, such as Zn, Pb, In and Sn, from formamide, acetamide,
glycerol, ethanol, acetone etc. should be mentioned. 38 The results do not
show any advantage over electroplating of these metals from aqueous
solutions. Usually, deposition of these metals from non aqueous solutions
produces deposits of poor quality and maintaining of solution chemistry
encounters experimental difficulties.

9. Electroplating and Surface Finishing

223

Electrodeposition of metals from liquid ammonia has also been studied
(Ag, Cu, Pb, Hg etc.). However, no results of a practical value were obtained.
Any metals from this group can readily be deposited from aqueous solutions.

Electrodeposition from non-aqueous electrolytes is attractive for several
metals and alloys that cannot be deposited from aqueous solutions.
Significant results have been obtained with aluminum and its alloys.
Improvements in electrodeposition from non-aqueous electrolytes will
continue to grow due to desirable physico-chemical characteristics of
coatings (e.g., aluminum or its alloys) or reactivity of substrate with aqueous
solutions (e.g., uranium). Developments in other areas of technology (energy
conversion, advanced batteries, electronics etc..) may lead to requirements
for materials with specific properties and to advancement in the field of
electrodeposition of metals and alloys from non-aqueous electrolytes.

9.5

ELECTROPLATING FROM ROOM
TEMPERATURE MOLTEN SALTS

As with plating from non-aqueous electrolytes, electrodeposition of metals
from molten salts has attracted the attention of researchers from two different
reasons. The first is related to the search of platability of metals from molten

224

Chapter 9

salts, especially those that cannot be plated from aqueous solutions or nonaqueous types of solutions. For those metals which are platable from aqueous
solutions e.g. zinc, the search for a convenient molten salts electroplating is
often related to a target to avoid the simultaneous hydrogen evolution reaction
and to obtain coatings of more desirable properties. In the electrodeposition of
zinc from aqueous solutions, the simultaneous hydrogen evolution reaction
significantly affects the embrittlement and sometimes reduces current
efficiency of the process. As a result an aprotic-plating bath is required.
Although there are well-established molten salts electrolyses, particularly
those related to the electrowinning of metals such as aluminum, magnesium,
sodium etc., attention in this section is directed towards the room
temperature molten salts electrodeposition. In this case, the hydrogen
reaction can be avoided, or deposition of metals that are not platable from
aqueous solutions may occur. The room temperature molten salts are based
on anhydrous
They are analogous to the high temperature melts
with a difference that the NaCl is replaced with an aromatic
organic chloride. This replacement of NaCl results in lowering the melting
point well below room temperature, sometimes as low as â&#x20AC;&#x201C; 50 Â°C. Several
types of organic aromatic chlorides have been investigated. This includes 1methyl-3-ethyl-imidazolium chloride (MEIC), l-(l-butyl) pyridinium
ammonium chloride (BPAC), 1, 2-dimethyl-3-propyl-imidazolium chloride
(DMPIC) etc. The most studied aromatic organic chloride for the room
temperature molten salts has been the MEIC. Room temperature molten salts
can be obtained from the combination of anhydrous
and MEIC. 46
These chloroaluminate salts have a high ionic conductivity, good thermal
stability, a wide electrochemical window and adjustable Lewis acidity. The
mixtures are liquid at room temperature (about 25 Â°C) over the
range of composition from 40 to 67 mol. %
In the acidic melts, metal ions are believed to be only weekly complexed
or solvated by
ions and thus can be reduced at more positive
potentials. These melts contain a molar excess of
In the alkaline
melts, the metal ions are strongly complexed by chloride ions and exist as
discrete anionic chloride complexes, e.g.
where z is the valence
of the metal ion. The alkaline melts contain a molar excess of MEIC. The
formation of strong anionic chloride complexes in alkaline region makes the
metal ions more difficult, or in some cases impossible to reduce within the
electrochemical window of the melt. Consequently, most of the studies on
the electrodeposition of metals from room temperature chloroaluminate
melts are carried out in the Lewis acidic composition region of the melt,
which contains a molar excess of
The Lewis acidity of
and organic chloride donor (RCl) mixtures is
a function of the
molar ratio, N, according to the equation:

9. Electroplating and Surface Finishing

225

Melts with the ratio N>1 (molar excess of
are Lewis acidic, while
those with N<1 (molar excess of RC1) are Lewis basic. The equilibrium
constant for the above equation is estimated from the potentiometric
titration46 at
In acidic melts, the dominant anionic species are
and
The electrochemical potentials are determined by
oxidation and
reduction. Deposition of aluminum proceeds according
to the reaction:

Pure metals such as palladium, gold, tin, mercury, lead and zinc, have
been electrodeposited from the acidic chloroaluminate melts. 47 The
electrodeposition of some transition metals is complicated by the
codeposition of aluminum. The formation of alloys such as Co-Al, Cu-Al,
Ni-Al, Cr-Al, Fe-Al is observed at potentials several hundreds milivolts
more positive than the potential at which the bulk deposition of aluminum
occurs. 48-52
Experiments are performed in a glove box with an inert atmosphere
(usually nitrogen). To remove the protonic or other impurities, the
melt is purified by pre-electrolysis of the melt at constant current
density for several days, while the electrolyte is stirred. The electrolyte is
then filtered to remove aluminum particles, which are produced during the
pre-electrolysis step.
Metal-ions (i.e. Zn, Co, Cu etc.) are introduced in the melt by an anodic
dissolution of corresponding metals. For the electrodeposition process
materials such as glassy carbon, platinum, tungsten etc. are used as working
electrodes (cathodes). To maintain the concentration of the metal ion in the
melt constant, the counter electrodes (anodes) are usually prepared from the
same metal. For the electrochemical measurements, the aluminum reference
electrodes are commonly used.
The current efficiency of the plating process frequently achieves 100 %.
The surface morphology of deposited films can vary from smooth and dense
to nodular, porous and dendritic deposits, depending on the experimental
conditions.
Due to experimental difficulties there are not, at the present, commercial
applications of the room temperature molten salts for the electroplating
purposes. Although very promising results are obtained by far, significant
amount of research and engineering should be carried out in the future, in
order to apply these processes on industrial scale.

226

9.6

Chapter 9

ELECTROPOLISHING

Electropolishing is defined as a process of anodic dissolution of metals or
alloys in an appropriate solution resulting in production of improved
morphology and geometry of the surface and a shiny, bright and smooth
appearance. Technical advantages of the electropolishing include a reduction
in coefficient of friction, an increase in the magnetic susceptibility of some
magnetic materials, an increase in corrosion resistance and excellent
reflectance. In addition, electropolishing is widely used in the metallography
for the microscopic investigation of crystallographic structure of metals and
alloys. Some theoretical aspects of electropolishing are discussed in the
section 3.2.3.
Electropolishing as an anode process, is similar to electromachining,
however there are significant differences between them. For instance,
electropolishing is usually carried out from unstirred, concentrated acidic
solutions as electrolytes, at lower current densities, and with the electrode
separations of at least 1 cm. The quality of electropolished surfaces depends
on electrochemical conditions including anodic polarization, electrolyte
composition and microgeometry. It is determined by the appearance,
measurements of profiles with optical profilometers, and also using
microscopic techniques. In terms of the surface roughness, the two types of
electropolishing are distinguished. The first, commonly called anodic
levelling or smoothing, refers to the elimination of the surface roughness
with a height of more than
The second type is called anodic
brightening and is referred to the elimination of surface roughness less than
However, this distinguishing between the smoothing and brightening
is a very approximate simplification, since there is no simple relationship
between measurements of profile and brightness.
A schematic presentation of the anodic current density â&#x20AC;&#x201C; potential
relationship, during the electropolishing process is given in Figure 9.9. Four
distinguishable regions on this curve can be seen: AB, BC, CD and DE. In
the region AB, the anode dissolves. Under these conditions the surface
microroughness does not disappear. The electropolishing takes place under
mass transfer conditions, at limiting current density and in the potential
range between and
The surface of the electrode becomes smooth and
microroughness decreases. When the potential approaches the value of
the surface of the metal becomes bright. An increase in the potential above
leads to a rapid increase in the current density, causing in this way an
increase in the roughness of the electrode surface, due to metal dissolution
and simultaneous oxygen evolution reaction.

9. Electroplating and Surface Finishing

227

A decrease in the microroughness during electropolishing is a
consequence of current distribution at the electrode surface. The surface of
an electrode profile, with distinguished protrusions and depressions is
schematically presented in Figure 9.10. It seems that the protrusions are
more accessible to the current flow than depressions. Therefore, under the
conditions of electropolishing, protrusions will dissolve faster, thus leading
in this way to a decrease in the microroughness. Thermodynamically, it is
more probable that the preferential transformation of solid phase (i.e., metal
crystals) into the ionic (solvated) phase would take place at protrusions than
in depressions. This is due to smaller energy of transformation of ions from
solid to solvated phase at protrusions than in the depressions. In practice, the
anodic dissolution frequently deteriorates microtopography due to unequal
localized etching.
The formation of a passive oxide layer at the anode surface plays crucial
role during the electropolishing. 53-55 Electrolytes used for electropolishing
contain substances (for example
which form an oxide passive film,
and acids, i.e.
or
which dissolve this passive film. Typical
formulations used in the electropolishing of aluminum, copper and stainless
steel are given in table 9.3.

228

Chapter 9

Electropolishing is carried out in concentrated aqueous solutions of
phosphoric acid, sulfuric acid or their mixtures, and sometimes in
combinations of perchloric and acetic acids. There are also formulations in
which, methanol is used instead of water.56
If the rate of passive film formation is less then the rate of dissolution, the
anode surface is rather etched, leading to an increase in the microroughness,

9. Electroplating and Surface Finishing

229

due to non-uniform coverage with oxide passive film. On the other hand,
when the rate of passive oxide film formation is more than the rate of
dissolution, the film thickness increases covering the anode surface. In this
way the electropolishing is not achieved. It is obvious that the
electropolishing occurs when the rates of the passive film formation and
dissolution are comparable.
The levelling of the electrode surface during electropolishing is a
consequence of a non-uniform passivation of protrusions and depressions. It
seems that the depressions are better covered with passive films than the
protrusions, leading in this way to faster dissolution of protrusions than
depressions. Passivation of protrusions to a lesser degree is explained by an
increased chemical activity, due to a faster rate of diffusion of metal ions on
protrusions than on depressions.
The rate of anodic levelling is equal to the difference in dissolution rate
between protrusions and depressions on a rough surface. It is determined by
the predominant current distribution on the surface profile. Consequently,
the rate of anodic levelling is dependent on geometrical, electrochemical and
hydrodynamic parameters.53,57
While the influence of the geometry of the surface remains qualitatively
the same, the charge transfer overvoltage (secondary current distribution)
tends to reduce the rate of anodic levelling. 57 When the concentration
overvolatge is present (tertiary current distribution), two cases are
distinguished. Below the limiting current, potential and mass transport
influence the current distribution, and, therefore, the rate of levelling. This
situation is of a little interest for the practical applications. At the limiting
current density, the current distribution depends only on mass transport.
For the anodic levelling and anodic brightening terms macrosmoothing
and microsmoothing were introduced by Edwards.53 The macrosmoothing
results from local differences in the current distribution on the surface profile
or the concentration of the transport limiting species, and is preceded by
microsmoothing. Macrosmoothing is a consequence of a higher current
distribution on protrusions, which causes higher distribution rates.
Microsmoothing results from the surface kinetics and passivation
behavior, due to suppression of the influence of surface defects and
crystallographic orientations on the dissolution process. The microsmoothing
occurs when dissolution of metal is mass transport controlled, which
corresponds to the limiting current plateau. In most electropolishing systems,
the rate of transport of dissolution products from the anode into the bulk
solution determines the limiting current.
The rate of levelling at the limiting current density may theoretically be
predicted on the basis of Nernst diffusion layer model. The Nernst diffusion
layer, for a triangular profile with the height,
is presented in Figure

230

Chapter 9

9.11. The broken line represents the outer limit of the Nernst diffusion layer
for an ideal microprofile. In an ideal case, when
the diffusion layer
should follow the surface profile as indicated by the broken line. Under these
conditions, the current density is uniform and only the geometrical levelling
should occur. However, due to local hydrodynamics, the situation is more
complicated, since the ratio between and can change significantly. For
this case a detailed modelling of the hydrodynamic perturbation, caused by
the surface microprofile is required for a quantitative prediction of the rate of
levelling.

In the aqueous solutions, during the high dissolution rate, the surface
concentration of dissolution products is in a reasonable agreement with the
saturation concentration of the corresponding metal salt. In the concentrated
acid type solutions
the surface concentration of products of
dissolution significantly exceeds the saturation concentration, which
probably causes formation of metastable species at the anode surface. During
the electropolishing, a low water concentration may reduce the limiting
current by decreasing the saturation of metal ions produced due to
dissolution.
There is general agreement among researchers that electropolishing
occurs when the reaction rate is controlled by mass transfer. In an attempt to
explain the electropolishing process the following mechanisms appeared in
the literature:
(i)
(ii)

salt â&#x20AC;&#x201C; film mechanism, and
acceptor mechanism.

9. Electroplating and Surface Finishing

231

The salt – film mechanism is based on an assumption that the surface
concentration of the metal ions due to dissolution is very high that exceeds
solubility and causes the precipitation of a salt film. The rate of reaction is
then determined by the rate of diffusion of metal ions away from the
electrode surface.
In the acceptor mechanism, the metal ions produced from the dissolution
remain on the electrode surface until they are complexed by an “acceptor”
species, which include either an anion or water. The rate of reaction, for this
case, is determined by mass-transfer of the acceptor to the electrode surface.
Consistent with the acceptor mechanism, the reactions describing dissolution
of copper may be presented as follows:

and

The experimentally obtained limiting current plateaus for electropolishing of
copper in concentrated phosphoric acid (85 % solution) at different rotation
speed are presented in Figure 9.12. 58 As this figure shows, the limiting
current plateaus extend over a potential range of 1 V. Above the current
plateau, the oxygen evolution reaction most probably takes place. According
to the results presented in Figure 9.12, the value of the limiting current
density depends on the rotation speed. An increase in the rotation speed
leads to an increase in the limiting current density.
In Figure 9.13 are presented dependencies of limiting current density on
the square root of rotation speed for temperatures 30 °C, 40 °C and 90 °C.
Based on their experimental results , Vidal and West supported the
acceptor mechanism theory, in which the limiting current density is given by
the Levich equation:

where is the limiting current density, F is the Faraday constant,
is the
diffusion coefficient of the acceptor species (probably water),
is the
kinematic viscosity, is the rotation speed and
is the concentration of the
acceptor species. They assumed that the concentration of the acceptor is

232

Chapter 9

constant over a wide range of temperatures. In this way, the validity of above
equation is experimentally confirmed by the results presented in Figure 9.12.
These authors rejected the salt-film mechanism, since it is unlikely that the
saturation concentration of salt is independent on temperature.

If the salt filmâ&#x20AC;&#x201C; mechanism were true the physical properties of this film
and its thickness are not well known. It is not clear if this film is a solid
oxide type film59 or an anhydrous film60. Difficulties in determining the
nature of this film arise due to its disappearance after the current is switched
off. The role of this salt film in microsmoothing is not clear yet.
The presence of a current plateau, associated with an anodic film on the
surface, is a very important condition for the microsmoothing. However, the
necessary condition for the microsmoothing is that the reaction rate is mass
transport controlled, which is achieved only in a specific concentration and
temperature range, as shown with well-defined current plateaus for
transpassive dissolution of nickel in sulfuric acid. 53

9. Electroplating and Surface Finishing

233

Anodic dissolution in the transpassive potential region below the limiting
current leads to crystallographic etching or pitting. The pitting is a local
attack of a passive metal, which is induced by certain ions under the effect of
high anodic potential. In order to achieve the uniform electropolishing the
pitting must be avoided. In systems where pitting may occur electropolishing
can be established with a sufficient increase in the anodic potential in order
to break down the passive film. Many electropolishing electrolytes are based
on a limited amount of water, which renders the formation of passive oxide
films more difficult.

9.7

ELECTROMACHINING

Electromachining is an electrochemical process in which metal removal is
achieved by the anodic dissolution. This process, frequently called

234

Chapter 9

electrochemical machining (ECM) is investigated as a method for shaping
high strength, heat-resistant metals and alloys, which are difficult to cut by
other established techniques. At the end of the last century, the
electromachining became a method employed in different industries including
automotive, offshore petroleum and medical engineering. Electrochemically
speaking, electromachining is a process very similar to the electropolishing,
since both processes are based on the anodic dissolution reactions. However,
the rate of metal removal for an electromachining process should be
considerably higher than that in the electropolishing. Therefore in the
electromachining, higher current densities are required, the electrode
separation is less than 1 cm and the process is carried out in diluted
electrolytes with stirring.
An anodic dissolution reaction is usually represented by:

The electrolyte and the material of the cathode are chosen in a way that
the cathodic process is usually hydrogen evolution reaction. Consequently,
the shape of this electrode does not change during the electromachining
process. Depending on the metal or alloy being electromachined, hydrolysis
reaction may occur due to dissolution:

To achieve reasonable results the precipitated hydroxides must be
removed from the electrolyte, and this is usually carried out by filtration.
A schematic presentation of electrochemical machining is given in Figure
9.14. During the electrolysis the cathode is moved simultaneously towards
the anode and a shape complementary to that of the cathode is produced on
the anode. The rate of the anode dissolution (metal removal) is in inverse
proportion to the distance between the cathode and the anode. Typical rates
of movement of the cathode towards the anode are about 0.02 mm/s.
A choice of the electrolyte in the electromachining process is crucial in
order to keep the shape of the cathode unchanged and to achieve desirable
current efficiencies of the process. This depends on the metal or alloy used
in the processing, and also on the nature of the cathode. The electrolyte is
usually pumped through the gap between the electrodes in order to remove
the products of machining (i.e., hydroxides and other solid particles,
hydrogen-gas bubbles accumulated at the cathode etc..), and also to reduce
the heating due to current flow.

9. Electroplating and Surface Finishing

235

The cathode, usually produced from a metal softer than the anode, with a
designed complementary shape is used as a tool. A workpiece is the anode.
Electrolytes based on NaCl,
etc. are used for the
electromachining process. 61,62 The results show that the surface brightening
depends on the concentration and the temperature of the electrolyte. After
passing through the gap between the electrodes the electrolyte is carefully
filtered to remove the products of electrolysis, and then heated in a reservoir
to the working (electromachining) temperature. The gap between the
electrodes is estimated at about 0.8 to 0.4 mm. The rate of metal machining
does not depend on the hardness of the material (i.e. anode), and any types of
profiles can be reproduced on hard metals, without the wearing the tool
(cathode).
The rate of electrochemical machining can be determined on the basis of
Faradayâ&#x20AC;&#x2122;s laws. The mass of metal dissolved (removed), m, is determined
according to the equation:

236

Chapter 9

where M is the atomic mass of the metal, n is the number of electrons, F is
the Faraday’s constant, I is the current and t is the time. The average current
densities depend on metal, and their values are usually between 50 and
while the voltage is about 10 to 20 V. Typical tolerances of about
0.127 mm are reported, however, under special circumstances they can
achieve 0.013 mm, or even 0.002 mm under pulsating current conditions. 63
During the electromachining the formation of surface oxide films
frequently occurs. To break the oxide film, higher voltages should be
applied. Due to oxygen evolution reaction at the anode the gas bubbles
rupture the oxide film causing localized pitting. The process variables can
significantly influence the surface finish. The smoother surface finish is
generally observed with higher current densities or with higher velocities of
the electrolyte.
Electrochemical machining processes have various applications such as
smoothing of rough surfaces, hole drilling, full form shaping, electrochemical
grinding, electrochemical arc machining, biomedical engineering etc. 64-67

9.8

ELECTROCHEMICAL OXIDATION OF METALS

Electrochemical oxidation of metals is an anodic process in which thin
oxide films are produced. These oxide films may have different color and
attractive physico-chemical properties. Color and other properties of
anodically produced oxide films are determined by the conditions of
electrochemical oxidation, which include composition of the electrolyte,
temperature, current density, voltage and duration of the process.
Thin oxide films can be produced anodically on many metals. Metals of
interest include so-called “valve” metals (i.e., metals such as aluminum,
tantalum, niobium, titanium, zirconium etc., which form adherent electrically
insulating anodic films,). At the present, this process is commercially applied
only to aluminum. Anodic oxidation of aluminum is often called
“anodizing”. Oxide films produced on aluminum surfaces as a consequence
of anodizing, have a very good hardness, abrasion- and corrosionresistances and unique columnar and porous structure.
Applications of anodized aluminum include protection against corrosion
and abrasion, decorative surfaces which provide color and base for paints
etc. These anodized surfaces are used in aggressive environments, permanent
external and architectural constructions, automotive, aircraft and electronics
industries.

9. Electroplating and Surface Finishing

237

The anodic oxidation of aluminum is carried out under both periodically
changing and direct current conditions. Typical electrolytes used for the
electrochemical oxidation of aluminum are given in Table 9.4.
The most widely used electrolytes for anodizing of aluminum are based
on sulfuric acid. In this case tanks are lead-lined with lead acting as the
cathode. So called “hard anodizing” (where extremely hard and abrasionresistant coatings are required) is carried out using sulfuric acid solutions at
higher voltages (above 25 V) and lower temperatures (about – 5 to + 5 °C)
Oxalic acid anodizing is mainly used for the wear resistance applications,
while anodic films produced by the anodic oxidation in the chromic acid
solutions have excellent corrosion properties.
General reaction for the anodic oxidation of aluminum is presented as:

Depending on the electrolyte and conditions of electrochemical
oxidation, the reaction products may be:
i)
ii)
iii)
iv)

soluble in the electrolyte,
almost insoluble in electrolyte,
sparingly soluble in the electrolyte, and
moderately soluble in the electrolyte.

When the reaction products are soluble in the electrolyte, the metal is
dissolved until the solution is saturated with its ions. This type of reaction
occurs in strong inorganic acids and bases. When the reaction products are
almost insoluble (electrolytes based on borates or tartrates) strongly adherent
and practically non-conductive very thin oxide films are formed. Sparingly
soluble oxide films are usually produced in the electrolytes based on
sulfuric, chromic or oxalic acid. In this case, the film growth is accompanied
by its dissolution at the surface. The rate of film growth is obviously higher

238

Chapter 9

than the rate of dissolution. Pores formed in the film are wide enough to
permit continuous access of the current to the metal, which leads to its
further oxidation. Finally, when the reaction products are moderately
soluble, the electropolishing is also possible if a proper electrolyte is used
(e.g. addition of sodium hydroxide to a sodium citrate bath.68
The mechanism of anodic oxidation of aluminum is very complex by its
nature and still not well understood. Formation of thin oxide films and their
composition depend on the electrolytes and conditions of electrochemical
oxidation.
There is a general agreement among researchers that the anodic oxide
film mainly consists of anhydrous aluminum oxide, which is either
45
amorphous or in the
In the amorphous anodic alumina material,
cations are both octahedrally and tetrahedrally coordinated to
ions.
Fresh films formed at in
solutions are composed of amorphous
with a small amount of water (about 1 %) and 2 â&#x20AC;&#x201C; 16 % sulfate. 69
Coatings produced in oxalic or chromic acid solutions contain about 3 %
oxalate or up to 0.7 % chromate. When freshly formed porous type anodic
coatings are boiled (a process known as â&#x20AC;&#x153;sealingâ&#x20AC;?), the alumina is converted
to a crystalline monohydrate, by take-up of about 5 to 6 % water.
Formation of thin oxide film during anodization of aluminum is
schematically presented in Figure 9.15.
As shown in Figure 9.15., the anodic films formed on aluminum during
the anodic oxidation consist of two layers, an inner, thin, dense,
dielectrically compact and the outer thick, porous layer. The inner layer is
called the active, barrier or dielectric layer. The thickness of this film may
vary from approximately
to
and represents only 0.1 to 2 %
of the total film. The barrier layer formed in the anodic oxidation of
aluminum is similar to the natural oxide layer formed at the surface in the air
atmosphere. It is non-porous, and conducts current only because of faults in
the skeleton and the fact that is very thin. The thickness of the barrier coating
is proportional to the voltage of the cell and is given with the following
empirical formula:

where d is the thickness, U is the voltage and k is a constant, with an
approximate value of
It is assumed that the barrier type of coating is produced as a result of
migration of mobile species across the pre-existing air-formed film. The
precise nature of mobile species and their mechanism of transport is not
clear yet, however the results indicate that
ion engress and
70
ingress proceed through the air-formed film.

9. Electroplating and Surface Finishing

239

The thickness of the barrier layer is mostly influenced by the type of
electrolyte
etc.) and its concentration. A
decrease in the barrier film thickness is observed with an increase in the
concentration of the electrolyte, however the reasons for this are not clear yet.
During electrolysis, the outer film dissolves according to the reaction:

As a consequence a porous film is formed (Figure 9.15). In this way, the
two parallel reactions at the anode during electrochemical oxidation of
aluminum are described by the equations (9.35) and (9.37). The main
cathodic reaction during anodic oxidation of aluminum is attributed to the
hydrogen evolution.
Porous anodic films, with their relatively regular morphology
characteristics have received significant attention from researchers. A
change in the thickness of the porous layer with time is presented
schematically in Figure 9.16. This, typically obtained thickness vs. time
curve shows that after the initial increase, the film thickness rapidly
decreases. This decrease in change of thickness is a consequence of an
increase in film dissolution and oxygen evolution reaction.

240

Chapter 9

With an increase in
concentration, when other parameters (i.e.
temperature, current density etc.) are constant, the rate of dissolution of
oxide increases leading to a decrease in film thickness and an increase in the
porosity. Temperature shows a similar effect on the growth of oxide film and
an increase in porosity. In order to produce thicker films, electrolysis in
solutions should be carried out at lower temperatures. Another
approach is to use less aggressive electrolytes.
Theoretically, if the rate of electrochemical film formation is proportional
to the current density, the rate of chemical dissolution should be constant. In
this way, an increase in the current density leads to an increase in the film
growth. However, in practice an increase in the current density leads to an
increase in temperature and, consequently, to an increase in the rate of
dissolution.
During the anodic dissolution, the total charge is consumed by the
following processes:
(i)
(ii)
(iii)

oxygen evolution reaction,
oxide formation, and
transfer of aluminum ions into electrolyte, due to film
dissolution.

9. Electroplating and Surface Finishing

241

Depending on the nature of the electrolyte involved in the anodic
oxidation, various secondary reactions may take place at the anode, which
affects the properties of the oxide film. As results show, sulfate, chromate or
oxalate are incorporated in the barrier layer. Reaction mechanism for
incorporation of these anions is not yet clarified.
The structure of porous
formed during anodic oxidation of
aluminum is described as a close-packed array of approximately hexagonal
columnar cells, which contain elongated pores normal to the Al substrate
surface. The schematic illustration of the porous anodic film is presented in
Figure 9.17. The processes inside the barrier layer are of a electrochemical
nature, while the processes inside the porous layer are of a chemical and
physical nature. 71-72
Anodic films produced in sulfuric acid solutions are semi-transparent and
colorless. In this way, they provide very good substrates for coloring.
Coloring of anodized aluminum is usually carried out with inorganic and
organic compounds and also electrolytically. 73,74

242

Chapter 9

Inorganic coloring techniques are based on immersion of anodized
aluminum into solutions containing ions, which will produce a color. For
example, anodized aluminum is successively immersed into solution
containing cobalt acetate and then a solution of potassium carbonate, at 30 to
50 °C, for blue color. A successive immersion of anodized aluminum, first in
a solution of lead nitrate and then in a solution of potassium chromate, at 40
to 50 °C, is carried out to produce yellow color. A green color is produced by
an immersion of anodized aluminum in a solution containing copper sulfate
and then a solution of ammonium sulfide. Mechanistic aspects of coloring
are not well understood, but results suggest this process is based on
absorption and diffusion of anions and cations into pores.
Among organic compounds dyestuff such as Alizarin Red S, Chrome
Fast Orange R, Aluminum Black MLW, Aluminum Turqoise PLWS etc., are
used for coloring.
Electrolytic coloring is carried out from Ni(II), Cu(II) or Sn(II)
electrolytes. Under the conditions of direct current (d.c.), anodized
aluminum acts as the cathode. However, since d.c. processes are sensitive to
contaminants present in the electrolyte, electrolytic coloring is frequently
carried out under alternating current conditions. The counter electrode
materials include graphite, nickel, stainless steal and lead.
As mentioned earlier, the oxide films on anodized aluminum are porous
and they have very good adsorption properties. In order to improve the
protection properties films are treated with boiling water, chromate, Ni(II)
solutions etc., which causes a further hydration of oxide and sealing of pores.
Sealing is a process in which major pore blockage and an increase in
water content in the anodic films occurs. This preventive method is
practically very important for the improvement of properties of anodized
films.
In the sealing with non-aqueous sealants, organic substances provide a
physical blockage of the coating pores. For this process lubricating oil,
silicone polymers, electrophoretic coatings with pigmented paint etc. are
used. 75
Sealing based on aqueous solutions is more widely used in commercial
applications. In this process, after a careful water-rinse, oxidized parts are
treated in saturated steam at atmospheric pressure or in hot water, nickel
acetate, chromate and various cold sealing solutions.
While “as produced” oxidized aluminum contains about 0.5 %
during sealing the amount of water increases rapidly in the initial stages of
the process, and later much more slowly. The amount of water in the sealed
coating is estimated at approximately 8 to 13 %.
In the sealing process, the porous structure of the anodic films becomes
less distinct and pores are blocked starting at outer surface. The aluminum

9. Electroplating and Surface Finishing

243

oxide is transformed into various forms of hydrated oxides. The most
important form of these hydrated oxides is böhmite.
Sealing methods used in practice are divided into following groups:
i)
ii)
iii)

water sealing,
steam sealing and
sealing in metal-salt solution.

Sealing in water depends on temperature (usually the working
temperature is 96 to 100 °C), presence of various ions
etc.) and pH (between 5.5 and 6.5). 76 The time of sealing is about 2 to 3
minutes per
of coating thickness.
The steam sealing presents considerable engineering difficulties, and
there is no evidence for superiority of this process over hot water sealing. As
mentioned above, the process is carried out with saturated steam and at
atmospheric pressure, although some plants use very large chambers, which
are hermetically sealed and the work is steamed under pressure. 77
Sealing in metal-salt solutions is based on nickel acetate or potassium
dichromate formulations. These processes are carried out at elevated
temperatures (e.g. sealing in the nickel acetate solution at 75 - 80 °C, and
sealing in the chromate solutions between 94 and 98 °C).
There are other sealing systems that were developed in order to reduce
the higher costs associated with hot water, or environmental concerns related
to chromate solutions. Among other systems cold sealing in nickel fluoride
solutions, sealing in sodium silicate solutions, or use of ammonia vapor
under pressure, should be mentioned.78

Deposition of metals and alloys without an external current source is very
important in modern technology, especially in the production of new
materials for applications in electronics, wear and corrosion resistant
materials, medical devices, battery technologies, etc.
These processes supplement and in some cases replace electrodeposition
for several practical reasons. The solutions for deposition of metals without an
external current source have excellent throwing power and allow plating on
articles of very complex shapes and plating through holes. Deposits are denser
and exhibit better properties for corrosion and electronics applications. Other
important advantages of this type of deposition over electrodeposition include
applicability for metallization of non-conductive surfaces (glass, ceramics,
polymers, etc.) and the ability to selectively deposit thin metal films only on
catalyzed areas of the substrate. Finally, for this type of deposition, an external
current source is not needed.
It seems that all metals electrochemically depositable from aqueous
solutions can also be deposited chemically under proper conditions (bath
composition, pH, temperature, and corresponding catalytic surface), using
suitable reducing agents.
Table 10.1 presents a survey of metals and alloys that have been
deposited without an external current source hitherto. In the first group are
listed commonly deposited single metals such as Ni, Co, Cu, Ag, Au and Pd.
Other metals from this group do not have significant applications at the
present, but it should be noted that there are reports on their deposition in the
published literature.
The second group lists elements, which cannot be deposited alone.
However, they can easily be codeposited with nickel or cobalt. Typical
examples are Mo and W. The phenomenon is somehow analogous to
induced electrodeposition as Brenner defined it.
249

250

Chapter 10

The third group represents alloys based on the first and/or second group
of elements. These alloys have been deposited for various applications,
mainly in the electronics industry. There is a high probability that other
alloys, which are commonly electrodeposited, can also be electrolessly
deposited, but there is no published data so far.

10.1

BASIC DEFINITIONS

Brenner and Riddell were the first authors to introduce the term
electroless metal deposition when describing an autocatalytic process of
depositing a metal in the absence of an external source of electrical current.
Since there are other metal depositions from aqueous solutions that are
carried out without an external current, this process can be divided into three
main groups:
1. displacement deposition
2. contact deposition
3. autocatalytic deposition

10.1.1 Displacement Deposition

Displacement deposition is a heterogenous galvanic process in which the
noble metal ions are reduced and deposited at the surface of an active metal,
as a consequence of dissolution of that metal. The process is sometimes
called immersion plating, although this term is not a specific description, and
therefore should be avoided, or cementation. The overall displacement
reaction is quite simple1:

and involves the displacement half-reaction of a more active metal

10. Metal Deposition without an External Current

251

by a more noble metal,

Typical cementation systems in practice are Ag/Zn, Ag/Cu, Cu/Zn, Cu/Fe,
Cu/Al, Sn/Cu etc. The displacement reaction stops immediately after the
reduced metal (more positive metal) covers the surface of the immersed
metal (more negative metal). Accordingly, the thickness of the deposited
metal is always limited. The time of immersion is particularly critical for
achieving a uniform coating layer. Very often, the adhesion of the deposited
films is not as good as that of films prepared by electrodeposition or by
autocatalytic deposition. The displacement deposition differs from all other
plating processes from aqueous solutions without an external source of
electrical current, because it does not require a reducing agent. Because of
lower quality and thinner coatings, displacement deposition has found
applications mainly in the refining metals. To a certain extent, however,
there are other applications such as coatings for porcelain enamelling,
zincate coatings, decorative finishing, soldering, purification of electrolytes
before electrowinning, environmental purposes, etc..2
10.1.2 Contact Deposition

Contact deposition is equivalent to electrochemical deposition with the
exception that the current is derived from the chemical reaction and not from
an outside source. The metal on which deposition takes place, and the
auxiliary metal with which it is in contact, form a galvanic element. In this
galvanic element, the auxiliary metal acts as an anode and dissolves; the
other metal is a cathode. Consequently, the dissolved metal is deposited on
the cathode (metal on which deposition takes place) at a mixed potential.
The importance of contact deposition for industrial applications is
relatively small. Sometimes the process is used with autocatalytic Ni to
initiate Ni deposition on copper and its alloys.2 This is achieved by coupling
the Cu or Cu-alloys with Al, Fe or Ni. The contact deposition is applicable
only to a limited extent, and uniform thicker deposits cannot be obtained. On
the other hand, the constant increase of dissolved metal concentration in the
solution may cause instability of the solution.
10.1.2

Autocatalytic Deposition

Autocatalytic deposition is the most commonly used chemical method for
the deposition of metallic films from aqueous solution without an external
source of electrical current. The metal films are formed only on catalytically

252

Chapter 10

active surfaces without an external source of electrical current and by the
chemical reduction of metallic ions in an aqueous solution containing a
reducing agent. Autocatalytic deposition is defined as a process for
deposition of metallic films by a controlled chemical reaction that is
catalyzed by the metal or alloy being deposited.3 If the metal ion,
is
reduced by the reducing agent ion,
the process can be simply described
by the following reaction:

Although the term electroless deposition broadly describes all processes
of metal and alloy deposition without an external source of electrical current,
it should be noted that this term is commonly used the for autocatalytic
deposition process. Consequently, in this chapter, the term electroless
deposition is used only for the autocatalytic deposition processes.
The development of electroless deposition is mainly connected
with Ni or Cu deposition. However, other electrolessly depositable
metals and/or alloys such as Ag, Au, Co, Sn, AuSn, NiWP, etc. have
also been studied because of their important applications.

10.2

SOLUTIONS FOR ELECTROLESS DEPOSITION

All solutions for electroless metal deposition have many similarities, but
depending on the metal or alloy to be deposited, there are also some
differences. Typically, a solution for electroless metal deposition is consisted
of the following components:
(i)
(ii)
(iii)
(iv)

Table 10.2. presents sources of metal ions in electroless deposition of
common metals. Generally speaking, the metal ion sources can be any watersoluble salts such as sulfates, chlorides, acetates, cyanides, etc. The nature of
the metal ion source is usually determined by the stability of the solution,
properties of the deposited films, and also by environmental issues.
The majority of complexing agents used in electroless metal deposition are
organic acids or their salts, with a few exceptions of inorganic ions such as
or
Ammonia and
ions, in the case of nickel solutions,
are mainly used for pH control. The choice of the complexing agents is
dependent, first of all, on the nature of the metal ion used for deposition. The

10. Metal Deposition without an External Current

253

principal functions of complexing agents are: buffering action, prevention of
precipitation of hydroxides and salts, and reduction of the concentration of free
(aquo) metal ions. In addition, complexing agents affect the rate of reduction
and the properties of metal deposits. In some cases, complexing agents
apparently form strong complexes with metallic contaminants, thereby making
them less susceptible to react with reducing agents.

Complexing agents used for the electroless deposition of common metals
are listed in Table 10.3. Commercial solutions for nickel electroless
deposition operate in the pH range 4.5 to 6. The complexing agents are most
effective in this pH range. However, in the electroless deposition of Cu, Au,
Ag, Pd and in some cases Ni, solutions with pH > 8 are used.

The choice of reducing agent depends on conditions of electroless
deposition and, of course, on the metal or alloy being deposited, including
their physico-chemical properties. Use of reducing agents containing
phosphorus or boron leads unavoidably to the incorporation of these
elements, which can dramatically affect the properties of the metal deposit.
On the other hand, electroless deposition of pure metals is also possible
using reducing agents such as hydrazine or formaldehyde.
Hypophosphite is mainly used for the electroless deposition of Ni, Co, Pd
and their alloys. The deposits are not purely metallic as they usually contain
phosphorus. Utilization of hypophosphite in electroless metal deposition is

254

Chapter 10

considerably less than 100 %. The reduction reaction takes place only at
certain surfaces such as metals of the group VIII (Fe, Co, Ni, Rh, Pd, and
Pt). It also takes place on Au.
The most studied reaction among electroless processes is definitely
deposition of Ni with hypophosphite. The overall reaction for nickel
deposition with hypophosphite can be represented as:

Generally, if the concentration of hypophosphite is increased, the
phosphorous content in NiP alloy is increased.
Electroless deposition of Cu with hypophosphite is still doubtful. In the
presence of nickel, however, NiCuP alloy films have been deposited
successfully.4
Boron-containing reducing agents used in electroless Ni deposition are
mainly borohydrides and amine boranes. Deposits usually contain 90 to 99 %
metallic phase, depending on the composition of the solution and operating
conditions. The rest is usually boron and other occluded reacting agents. Theboron containing reducing agents are used for electroless deposition of
common metals, such as Ni, Co, Pd, Pt, Au, Ag and their alloys.
The electroless deposition of Ni with borohydride takes place in alkaline
solutions. Theoretically, each borohydride ion can reduce four nickel ions:

However, experimental results show that one mole of borohydride
reduces approximately one mole of nickel ion.
Gorbunova et al. investigated the conditions for electroless nickel-boron
deposition using sodium borohydride as a reducing agent.5 They found that
an increase in the
concentration in solution without a stabilizer or
with stabilizers such as lead chloride, 2-mercaptobenzothiazole or thallium
nitrate, leads to an increase in the rate of Ni-B deposition (Figure 10.1).
Using a solution containing
as a stabilizer gives a faster rate of Ni-B
deposition.
Using borohydride as the reducing agent, gold-based alloys (Au-Ag, AuIn) and metals such as Pt, In and Co were deposited.5
Whereas borohydrides such as
are completely ionic, the amine
boranes are covalent compounds. The electrons in the amine boranes are
displaced toward the boron atom, while the nitrogen atom displaces positive
charge as is illustrated by the following formula:

10. Metal Deposition without an External Current

255

In practice, the application of aminoboranes is limited to dimethylamine
borane,
Dimethylamine borane (DMAB) is used for the
electroless deposition of Ni, Cu, Co and Ag. In alkaline and neutral
solutions, the preceding chemical reaction of dimethylamine borane with
ions can be represented as:

The acid-catalyzed hydrolysis of dimethylamine borane occurs according
to the following equation:

Based on experimental results, in the electroless nickel deposition the
molar ratio of nickel ions reduced to DMAB molecules consumed during the
process is approximately 1:1.

256

Chapter 10

Parlstein and Weightman investigated electroless deposition of Co with
DMAB from acid solutions. 6 Dependence of deposition rate on DMAB
concentration is presented in Figure 10.2. As illustrated in this figure, the
rate of Co deposition increases almost linearly up to DMAB concentration of
about
A further increase in DMAB concentration results in a rapid
decomposition of the solution.

Formaldehyde is mainly used for electroless copper and silver deposition;
however, there are reports that this reducing agent can also be used for
electroless deposition of AuCu alloy or Co.
An overall reaction for electroless copper deposition with formaldehyde
is described as follows:

Dumesic et al. studied electroless copper deposition from an EDTA
alkaline solution using formaldehyde as a reducing agent.7 They reported

10. Meted Deposition without an External Current

257

that an increase in the formaldehyde concentration from 0.03 to
leads to a linear increase in the initial deposition rate (Figure 10.3.).

Electroless Ag deposition with formaldehyde is fast, but either a cloudy
film of silver metal is obtained or peeling occurs. From other metals, as
mentioned earlier, the electroless Co deposition is carried out using
formaldehyde as a reducing agent.8
Hydrazine has long been recognized as a very powerful reductant of
metallic ions, and has been used for electroless deposition of metals and
alloys. Examples include electroless Cu, Ni, Co, Au, Ag, Pt-group of metals
and their alloys, NiSnW, NiFe and alloys resembling stainless steel.
The rate of electroless deposition of Co with hydrazine, increases with an
increase in
concentration, which is presented in Figure 10.4. 9 The net
reaction for electroless deposition of Co with hydrazine is described as:

Similarly to other reducing agents, increasing hydrazine concentration
leads to an increase in the rate of electroless deposition.

258

Chapter 10

Hydrazine is often used in the spray method for mirror production as the
deposition rate is fast. The deposition of Ag from an
complex
solution can be described with the following reaction:

Stabilizers are chemical compounds used in electroless deposition of
metals in order to avoid the decomposition of the solution. Addition of these
compounds to the plating solution assures, under proper conditions,
operations over an extended period of time. Bath decomposition occurs as a
precipitation of metallic particles in the bulk solution. These particles act as
a highly efficient catalyst for further metal reduction because of their large
surface area. The choice of a stabilizer depends on the metal being deposited
and its compatibility with the process.
Stabilizers, used in the electroless deposition of Ni, have been divided
into the following classes:2

The concentration of stabilizers is very important since it determines the
rate of deposition. An increase in the concentration of stabilizers of classes I
or II above 2 ppm may completely inhibit the deposition reaction. The
concentration of class III stabilizers is in the range
to
and the concentration of class IV stabilizers is in the range
to

10.3 MECHANISTIC ASPECTS OF ELECTROLESS
DEPOSITION
In spite of relatively intensive study of the electroless deposition of
metals and alloys, there is still some disagreement in the treatment of
mechanistic aspects of these processes. In order to explain electroless
deposition of metals and alloys, five different mechanisms have been
proposed, as follows:
1.
2.
3.
4.
5.

These mechanisms involve various attempts to explain electroless
deposition. However, according to some experimentally observed
characteristics, it is difficult to use any one of these mechanisms for a general
explanation of an electroless deposition process.
10.3.1

The Atomic Hydrogen Mechanism

The atomic hydrogen mechanism was developed for electroless Ni
deposition with hypophosphite. Brenner and Riddell2 postulated that the
atomic hydrogen reduces
ions and acts by heterogenous catalysis at the
catalytic Ni surface. Atomic hydrogen is generated by the reaction of
hypophosphite with water, and is then desorbed at the catalytic surface
according to the equation below:

260

Chapter 10

At the catalytic surface, the adsorbed hydrogen reduces

ions:

The atomic hydrogen mechanism fails to explain many of the features of
electroless deposition such as the simultaneous hydrogen evolution reaction.
In this mechanism, deposition of phosphorus and involvement of hydrogen
evolution reactions are explained as side reactions. Furthermore, this scheme
does not explain why the stoichiometric utilization of hypophosphite is
always less than 50 %.
10.3.2 The Hydride Ion Mechanism
In the hydride ion mechanism, the hypophosphite acts as the donor of
hydride
ions. The hydride ion is the reducing agent of both
and
10
ions. This mechanism, was modified by Lukes who applied it to both
acidic and alkaline solutions. In acidic solutions, formation of the hydride
ion was described by the reaction:

In alkaline solutions, the formation of
following reaction:

Lukes described by the

Two hydride ions from the above reactions can then react with
one ion with either a hydrogen ion or water, to form Ni metal and

or

In broad terms, the hydride ion mechanism can then be described by the
following general equations:

10. Metal Deposition without an External Current

261

where RH is formaldehyde, hydrazine or hypophosphite as previously.
From Lukes’ theory arises a question of the reality of a hydride ion
formation having a standard reduction potential of - 2.08 V in a
hypophosphite solution with standard potential of -1.57 V. 11 Both potentials
are reported for pH=0. The change-over from standard conditions to those in
which metals are reduced by hypophosphite does not alter the difference
between these potentials. On the other hand, the existence of hydride ions in
an alkaline medium, even in an intermediate state, appears very unlikely.
10.3.3

The Electrochemical Mechanism

The so-called electrochemical mechanism was first proposed by Brenner
and Riddell2 and later modified by other researchers. In this mechanism,
electroless deposition is considered to result from mixed anodic and cathodic
reactions. In the case of electroless Ni deposition, the oxidation of
hypophosphite with water generates electrons, and is considered as the
partial anodic process:

with E° =0.50 V.
The electrons formed in the above reaction are utilized in the coupled
cathodic processes for deposition of Ni and P:

with E° = - 0.25 V, and

with E°= - 0.50 V.
According to the electrochemical mechanism, the evolution of hydrogen
gas is a result of the secondary reaction, which follows:

with E°=0.00 V.

262

Chapter 10

The electrochemical mechanism does not explain reduction of metal ions
in the bulk solution (i.e. without the presence of a metallic substrate). It also
does not explain the reduction of metal hydroxides (formed as precipitates)
to a metallic state. As experimental results show, the presence of any
metallic surface is not a sufficient condition to start electroless deposition.
In terms of mixed potential theory, electroless deposition was first
described by Paunovic. 12 According to this theory, electroless metal
deposition can be considered as the superposition of anodic and cathodic
curves crossing at the mixed potential,
Electroless deposition of metals
takes place at the mixed potential. The mixed potential,
and the
deposition current,
are obtained by the intersection of the partial anodic
and cathodic polarization curves, as it is schematically shown in Figure 10.5.
This theory predicts that the rates of anodic reactions do not depend on
the cathodic processes occurring simultaneously at the cathodic surface. The
rates of separate reactions (anodic and cathodic) depend only on the mixed
potential at which they have the same values.
By the applying the mixed potential theory it was suggested that the
mechanism can be predicted from the polarization curves for the partial

10. Metal Deposition without an External Current

263

processes. However, the extrapolation of partial polarization curves and
application of the mixed potential theory is not often realized, since the two
partial processes are independent of each other. 13
103.4

Metal Hydroxide Mechanism

The metal hydroxide mechanism was originally proposed by Salvago and
Cavallotti. 14 This mechanism can be described briefly by the following
scheme.
At the catalytic Ni surface, the ionization of water takes place according
to the reaction:

Hydrolysis of
follows:

and formation of hydroxo-complexes takes place as

and

The hypophosphite ions interact directly with hydrolyzed species, as is
indicated below:

Deposition of phosphorus is explained in terms of the reaction:

The hydrolyzed Ni(I) species interact directly with water:

264

Chapter 10

Evolution of hydrogen can is explained as:

or by the reaction of hypophosphite ions with water:

According to the reactions (10.29) and (10.30) Savago and Cavallotti14
explained lamellar morphology of electroless NiP deposits. It is obvious that
any periodicity between the reactions (10.29) and (10.30) will produce
deposits having layers richer with P, and then layers richer with Ni (lamellar
morphology). Cavallotti and Savago reported that when nickel hydroxide is
precipitated, inhibition phenomena are evident.
Experimental results, obtained for electroless deposition of Co, using
hydrazine as the reducing agent, support the metal hydroxide mechanism. 9
The observations in this work9 support the metal hydroxide mechanism as a
means of explaining the electroless deposition of Co by hydrazine.
Hydrolyzed species of
can react directly with hydrazine producing
metal powder. The reaction occurs in bulk electrolyte, when a precipitate of
cobalt hydroxide is formed and production of Co powder takes place. The
SEM micrographs show that the Co powder produced under these conditions
was dendritic in terms of surface morphology (Fig. 10.6).

10. Metal Deposition without an External Current

265

XRD patterns in Figure 10.7. show that the Co powder contained the 70
% Co having the hcp structure and 30 % Co the fcc. Although the metal
hydroxide mechanism explains most of the characteristics of electroless Co
deposition by hydrazine, particularly the reduction of precipitated cobalt
hydroxides and deposition of dendritic Co powder, there are still some
doubts about this mechanism. For example, it does not explain the oxidation
of
at a Pd-activated surface in Co(II)-free solutions. In order to explain
9
deposition of shiny and smooth coatings at flat surfaces,
suggested
that contributions from both the electrochemical and the metal hydroxide
mechanisms should be considered.

103.5

The Universal Mechanism

Based on similarities among electroless processes, van den Meeraker
proposed a mechanism that accounts for both the electrochemical and the
catalytic nature of the process.15 This mechanism was developed according
to the following features, which are common to different electroless systems:
(a) The electroless deposition process proceeds only on certain catalytic
metals that are known as effective hydrogenation-dehydrogenation catalysts;

266

Chapter 10

(b) Electroless deposition is always accompanied by evolution of
hydrogen gas;
(c) Poisons for hydrogenation-dehydrogenation reactions, such as
thiourea and mercaptobenzothiazole, act as stabilizers in practically all
electroless processes; and
(d) The deposition rate increases with an increase in pH.
The reactions taking place during electroless deposition were described
as follows:
Anodic:
Dehydrogenation: RH = R + H
Oxidation:
Recombination:
Oxidation:
Cathodic:
Metal Deposition:
Hydrogen Evolution:
In this scheme, RH represents the reducing agent. It dissociates to a
radical R and atomic hydrogen. The electrons for reduction of metal ions are
supplied by the oxidation of R and/or reaction of H with
The universal mechanism is not adequate for the explanation of all
electroless processes. It fails to explain electroless deposition of metals on
non-conductive surfaces, and also deposition of metal particles in solution.
The proposed mechanisms explain most of the characteristics of electroless deposition. However, as discussed above, there are some characteristics,
which cannot be explained using these mechanisms. It seems that major
problems arise when attempting to generalize the proposed models for
electroless deposition. A more realistic approach would be to look for specific reactions, for particular conditions and substrates. It is very unlikely, in
spite of the similarities of electroless processes, that a general mechanism
will be developed explaining features for all electroless deposition of metals.

10.4 APPLICATIONS AND PROPERTIES OF
ELECTROLESS DEPOSITED FILMS
Development of electroless deposition of metals and alloys in the past
years has been remarkable and still continues. This process was investigated
for various applications such as magnetic disks, printed circuits, selective plating on semiconductors, batteries, medical devices, etc. Most of these applications are related to electroless deposition of copper or nickel. Considering the

10. Metal Deposition without an External Current

267

fact that many of these applications use similar approaches and in a way
overlap, the further discussion in this section is presented as follows:
1. metallization of non-conductive surfaces
2. electroless deposition of composite coatings
3. electroless deposition of gold and other metals

10.4.1

Metallization of Non-Conductive Surfaces

Metallization of non-conductive surfaces (polymers, ceramics and glass)
requires specific treatments prior to electroless plating. Usually, these
surfaces are first etched, then sensitized by a simple immersion in a
solution. During the sensitization process, the adsorption of
ions takes place. Senzitized surfaces are then exposed to a solution
containing
and HC1. This process is called activation. The activation
process can be described by the following equation:

The Pd sites formed during the activation step allow chemical deposition
of Ni or Cu. In some cases, sensitization and activation steps are combined
in one step. In other words, solutions for the sensitization and activation are
combined and represent mixtures of
and HCl.
Other processes recommended for the activation of non-conductive
surfaces for metal deposition from aqueous solutions (electroless deposition
or electrodeposition) are carbon/graphite systems, conductive polymers, and
non-formaldehyde based electroless processes.
For applications where metallization of advanced devices on nonconductive substrates takes place, Cu, Ni, Ag etc. are deposited by chemical
or physical vapor deposition. These substrates are then ready for further
metallization by the electroless deposition.
Metallization of polymers and plastics has also attracted significant
attention from researchers because of its various industrial applications.
Polymers of interest included polyimides, polystyrene, polycarbonates,
polyetherimides, ABS plastics, aramid fibres, fluoropolymers etc.
10.4.2 Electroless Deposition of Composite Coatings

Codeposition of solid, inert particulates within a metal matrix during
electroless deposition of that metal matrix (single metal or alloy) leads to
production of composite coatings. In typical composite coatings, the fine
particulatesâ&#x20AC;&#x2122; diameter size ranges from 0.1 to
Their content in the

268

Chapter 10

coating can exceed up to 40 vol.%. The metal matrix in this class of
composite coatings is usually electroless NiP or NiB. These materials are
used for the improvement of wear and corrosion resistances, friction
coefficient and hardness. Inert particulates, depending on applications, may
include chromium carbide, alumina, titanium carbide, silicon carbide, boron
carbide, diamonds, PTFE etc.
There is renewed interest in coatings with exceptional hardness, wear and
friction properties for automotive and other mechanical applications. High
performance coatings include alloys of cobalt and tungsten, composites with
fluoropolymers etc. These coatings have potential for replacement of hard
chromium.
Commercial acceptance of composite coatings has increased in a number
of applications, especially since productivity, quality and environmental
concerns continue to expand at increasing rates.
10.4.3

Electroless Deposition of Gold

Electroless deposition of Au is used for applications in the electronics
industries (deposition on semiconductors and circuit patterns), as well as for
decorative purposes. Solutions for electroless deposition and properties of
deposited Au films were reviewed recently.16
The classical electroless Au solutions utilize
as a source of Au
and
or DMAB as reducing agents. These solutions are autocatalytic,
and it is possible to deposit sufficiently thick layers of gold.
In order to replace cyanide-based solutions, two different systems for
electroless deposition of Au were developed in recent years. Both use gold
thiosulfate as their source of gold. 17 The main difference is the reducing
agent. The first system uses thiourea, while the second uses ascorbic acid.
Deposition of Au using ascorbate as the reducing agent, is explained as
the combination of an anodic reaction (oxidation of ascorbic acid):

coupled with the cathodic reaction (reduction of Au(I) to Au):

In deposition of Au with thiourea as the reducing agent, the anodic
reaction is:

10. Metal Deposition without an External Current

269

while the cathodic reaction is described by equation (10.36).
Other developments include improvements of non-cyanide solutions
(gold(I) thiosulfate with ascorbic acid as a reducing agent) for the electroless
deposition of gold, which prevents the formation of any precipitates during
the storage of the bath18, and solutions with chelating agents, such as
diethylenetetraaminepentaacetic acid19, dimethylamine20, etc.
10.4.4 Electroless Deposition of Other Metals

Other metals, studied from the aspect of electroless deposition, include
Ag, Sn, Sb-Pb, Bi, Sn-Bi solder, Pd, Ni-Sn-P alloys, etc.
It is first reported by Rutkevich et al., that Bi can be reduced by Ti(III)
complexes in an autocatalytic mode. 21 The main characteristics of the
process are explained in terms of pure electrochemical mechanism. The
reduction of Bi(III) to Bi is represented as:

The authors also claimed a possibility of using Ti(III) complexes to reduce
Ni(II) and Co(II), as well as application of V(III) for autocatalytic Cu(II)
reduction.
NiSnP and NiSnB alloys are deposited from alkaline solutions containing
as a complexing agent, using sodium hypophosphite and DMAB
as reducing agents, respectively. For electroless deposition of NiSnP a
source of Sn is
and for NiSnB, a source of Sn is Sn(IV) gluconate.
The maximum contents of Sn in the deposit are estimated at 30 at.% for
NiSnP and 42 at.% for NiSnB alloys. The crystallinity of alloys increases as
the Sn content increases.
In spite of development of other competitive technologies, it is
obvious that applications of electroless deposition of metals and alloys
will continue to grow in the future. More work is required to
understand fundamental issues related to the reaction mechanisms of
electroless deposition, the influence of processing parameters on
properties of deposited coatings, etc. This knowledge is needed to
ensure the successful operation of the process.
The new electroless deposition-based technologies should improve
selectivity, satisfy quality requirements of deposited coatings, and assure the
consistency of the process. The environmental concerns related to the
solutions used for electroless deposition must also be investigated, in order
to develop environmentally-friendly technologies, and to allow successful
competition with other available processes.

Molten salts electrolysis is an attractive method for production of metals
that cannot be deposited from aqueous solutions. Although there are many
metals which can be deposited from fused-salt systems, on the industrial
scale this method is used for the production of only a few metals.
Molten salts electrolysis is used for electrodeposition of alkaline and
alkaline earth metals, Al, Ti, Zr, Ta, Mo, W etc. Among these metals,
aluminum, magnesium, alkaline and some refractory metals have the most
significant industrial importance from an electrometallurgy point of view.
Sodium is won by electrolysis of the fused hydroxide (Castner process),
or a fused mixture of NaCl and
(Down process). At the present, in
western countries the electrolytic production of calcium is completely
replaced by the alumothermic processes. However, in China and Russia, it is
believed that calcium is produced by the electrolytic method.
Magnesium is usually produced by electrolysis of the molten electrolyte
containing alkali metal and magnesium chloride. In this process anhydrous
as the feed material is used, which requires a relatively high cost.
More recently, electrowinning of magnesium from MgO in a melt containing
neodymium chloride is suggested.1 This process is based on the reaction of
MgO with
Magnesium is then electrowon from a resulting
melt. Aluminum is produced by the electrolysis of a
molten cryolite, in which alumina (as a source of aluminum) is dissolved.
In Table 11.1 are presented some metals, electrowon from their fused
salts. This table includes Li, Na, K, Mg, Mn and Al, although manganese is
at the present mainly produced by the electrolysis of aqueous-based
electrolytes.
Other metals that can be produced by the electrolysis from their fused
salts include Be, Ti, Ta, Nb, W etc.
271

272

Chapter 11

Procedures for deposition of beryllium involve electrolysis of
in a
variety of fused salts. Tantalum or niobium can be produced by electrolysis
of chloride-fluoride melts such as LiF-NaF-KCl-NaCl at 700 째C, in which
or
are added.2,3 The refractory metals, such as for example
tungsten, can be produced from
or
melts containing
4
or
at 350 to 450 째C.

Principally, the electrolysis of molten salts is very similar to the
electrolysis from aqueous solutions. However, there are significant
differences.
Decrease in the current efficiency during electrowinning from fused salts
is often observed, and attributed to the following factors:
(i)
(ii)
(iii)
(iv)

evaporation of the products of electrolysis,
appearance of secondary reactions,
dissolution of metals in fused salts, and
dissolution of anodic products in fused salts.

In order to reduce the evaporation of the products of electrolysis, the
process is usually carried out at lowest possible temperatures. The
appearance of secondary reactions may be avoided using proper cell design
and proper electrode materials. The most significant decrease in the current

11. Electrodeposition of Metals from Molten Salts

273

efficiency is related to the dissolution of the cathodic and anodic products in
the melt, their diffusion into the bulk electrolyte, formation of intermediate
compounds and oxidation of metal dissolved with the oxygen from air.
Molten salts electrochemical studies include aspects of melt handling, cell
design, materials choice, selection of electrodes, properties of melt, its
purification etc. Purification of melts is a very important operation and it
always depends on the nature of a melt involved in the study. For example,
water is a very critical contaminant of lithium and magnesium based melts. It
is usually removed by long-term drying over
followed by pumping,
but only in the range of low concentrations. On the other hand
could
be a very dangerous contaminant of alumina-cryolite melts. With higher
concentrations of water more sophisticated techniques are required. Another,
commonly used pretreatment is pre-electrolysis. This step is usually applied
for removal of some heavy metals and is carried out carefully to avoid
undesirable side reactions of some anionic impurities.
Materials employed in studies or industrial processes are selected on the
basis of the nature of a melt and operating conditions. The use of dried and
deoxygenated, inert atmosphere is a general requirement in order to reduce
oxidation and corrosion as well as to prevent the generation of electroactive
oxygen-containing species in the melts. Crucibles are usually made of
ceramic materials, graphite and refractory metals.
Reference electrodes in molten electrolytes have considerable problems
related to the very strong ionic interactions at high temperatures.
Consequently, redox series can only be defined for single melts.
Since metals are electrodeposited on cathodes, the choice of cathodes is
an important factor for normal electrolysis. In the many instances, various
carbon materials or stainless steel are used as the cathode. Studies on
electrodeposition of metals from fused salts at electrodes such as platinum,
gold, tungsten and molybdenum have also been performed in spite of
difficulties associated with their use.
In the electrodeposition of metals molten salts, counter electrodes
(anodes) are very important theoretically and practically. They will be
discussed later in a separate section, but it is to be noted that a choice of an
anode material depends on the melt composition and electrolysis conditions.
Anode materials used for this purpose include various types of graphite
materials, platinum, gold, refractory metals etc.

11.1

IONICALLY CONDUCTING MELTS

Electrolytic conductivity of molten salts is a very important property
from both theoretical and practical points of view. This information is useful
for a better understanding of the mechanisms of the transport processes and

274

Chapter 11

electrodeposition of metals from fused salts, and also for the reduction of
ohmic drop through the electrolyte and an improvement of the efficiency of
the electrowinning process.
The electrical conductivity,
of a molten electrolyte is related to its
resistance, R, according to the expression that follows:5

where G is the cell constant defined as the ratio of length to area. In an
uniform electric field with a potential gradient of 1 V/cm, the specific
conductivity is determined by the concentration of each ionic species
the
charge of each ion
and the mobility
i.e.,

where F is the Faradayâ&#x20AC;&#x2122;s constant. In the case of binary, fully dissociated
electrolyte, can be written as:

This equation can be rearranged to:

where
and
are the ionic conductivities of the anion and cation,
and is the equivalent conductivity of the melt. The electrical conductance
depends on temperature according to the basic Arrhenius type equation:

The relationship between

and

is given as:

11. Electrodeposition of Metals from Molten Salts

275

where is the coefficient of expansion. This equation shows that the two
forms of the activation energy are equal only for systems with very small
temperature coefficients of expansion (for example silicate melts) or at
relatively low temperatures (the maximum temperature suggested is about
125 째C.
Measurements of electrical conductivity are similar to those in aqueous
solutions, however it should be noted that temperature and corrosion
resistant materials must be used. Combined with other transport properties
(viscosity, diffusivity, thermal conductivity) and their temperature
coefficients, the electrical conductivity is of particular value for postulating
mechanisms for the transport processes in terms of lattice geometry,
molecular force fields and molecular motion.

11.2

ELECTROCHEMICAL STUDIES IN
ELECTRODEPOSITION FROM MOLTEN SALTS

The electrochemical measurements are not basically different from those
in the aqueous solutions. However, practical difficulties often arise from the
fact that these systems are very corrosive and that measurements should be
carried out at relatively high temperatures, above 600 째C for magnesium
chloride melts and about 1000 째C for the alumina-cryolite melts.
Electrochemical studies of molten salts systems have led to determination
of important parameters such as reversible potentials, diffusion coefficients
etc., which advance fundamental understanding of molten salts electrolysis
and facilitates process control. However, there are some discrepancies in the
published results. These discrepancies arise due to extremely difficult
experimental conditions (high temperature and very corrosive environment).
Electrochemical techniques are easily applied for studies of molten salts
since they have very low ohmic resistance, the interference from surfaceactive materials is relatively small and the rates of the charge transfer
reactions and secondary chemical processes are high. Most of the reactions
in the molten salts systems are under mass-transfer conditions.
Electrochemical studies in electrodeposition from molten salts are very
useful for obvious reasons. They allow an in-depth understanding of anodic
and cathodic reactions involved in the process. The knowledge gained
through these measurements is important for a better maintenance and
control of industrial cells, an increase in the process efficiency, etc..
Almost all electrochemical techniques developed in the aqueous
solutions, with specific adjustments, have been used in the molten salt
electrochemistry for studying electrodeposition of metals. Among these
techniques, steady state, open circuit potential, chronoamperometry,
chronopotentiometry, cyclic voltammetry, a.c. impedance etc., should be

276

Chapter 11

mentioned. The application of these methods requires specific attention on
choice of materials, which have to be able to withstand extremely corrosive
conditions and elevated temperatures. These electrochemical processes have
been used in studying of both, cathodic and anodic processes.
11.2.1

Electrolysis of Alumina-Cryolite Melts

Taking into consideration the fact that the most important metal produced
by the electrodeposition from molten salts is aluminum, further discussions
in this chapter are mainly restricted to the achievements related to this field.
The principal basis for production of aluminum by the Hall-Héroult
process can simply be described in terms of the following reactions, for
which the respective thermodynamically calculated E° values are indicated
as shown:

E° =1.163 V at 1010 °C
and

E°=1.024 V at 1010 °C
The electrolyte consists of fused cryolite
in which alumina
in the concentration range from 2 to 6 wt.% is dissolved. The melt
always contains about 4 to 8 %
arising from low levels of calcium
oxide impurities in the alumina. The concentration of
in the melt is
adjusted automatically in the smelter cell by means of periodic mechanicalfeed system. The relative ratio of molar contents of NaF and
in the
electrolyte is the so-called cryolite ratio. Industrial cells operate at cryolite
ratios between 2 and 3 and temperatures from 940 to 980 °C.
Ionic conductivity of alumina-cryolite melts arises from the migration of
toward the cathode and complex alumofluoride anions toward the
anode.6 There is no evidence of existence of any uncomplexed aluminum
cations.
The primary ionization of cryolite melt is represented by the
following equations:

11. Electrodeposition of Metals from Molten Salts

277

The degree of dissociation of hexafluoraluminate ion at the melting point
of cryolite is about 0.3. In this way, cryolite melts contain mainly
and
The addition of alumina to a cryolite melt and a
consequent
dissolution cause properties of the melt such as surface
tension, vapor pressure, electrical conductivity and density to change
rapidly, with a tendency to decrease while the viscosity increases with
increase of alumina content.
The nature and the number of the ionic species formed in the solution of
in cryolite are still incompletely understood, although this matter has
been studied by means of a variety of modern physico-chemical methods.7
The complex oxyfluoroaluminates, with a general formula
are probably formed in the dissolution process of alumina in cryolite, rather
than simpler aluminates such as
The effective cathodic reaction in the electrolysis of alumina-cryolite
melt is reduction of
ions and production of aluminum metal:

with the following side reactions:

The real situation is, however, much more complicated. Taking into
account the fact that the free
ions are not present in the melt as well as
the fact that
ions are the principal current carriers it seems that the
discharge of
could be the primary process at the cathode:

and discharge of aluminum metal results from secondary reactions. In
industrial cells, however, the cathodic product is mainly aluminum, with
sodium present at very low activity. It appears that the cathodic process can
be considered as a reversible three-electron transfer. One possible

278

Chapter 11

mechanism for the cathodic discharge of
ions is based on the hypothesis
that this process is proceeded by dissociation:

In the existing literature, however, there is no evidence for a chemical
reaction preceding electron transfer. Other three-electron-transfer processes
are also possible, such as, for example, a process involving
oxyfluoroaluminate ions. In Houpins and Frakâ&#x20AC;&#x2122;s opinion6, the most probable
cathodic reactions are:

and

Anodic processes in the electrolysis of alumina-cryolite melts include the
discharge of oxygen-containing species and consequent formation of CO and
The primary gas found at carbon anodes is
although chemical
analysis shows that only about 60 % of the gaseous products is
This is
a result of secondary process that arise on the account of the reaction of
aluminum metal with
forming
and CO.
The anode processes probably involve electrosorptive formation of
oxygen-carbon compounds to CO and
and their desorption from the
electrode surface. In general, in the intermediate compounds
the ratio
x/y is a function of time, temperature, nature of the carbon anode material,
current density etc. The rate of CO formation is slow so that, at commercial
current densities, the composition of anodically produced gases approaches
about 100 %
The following anodic reactions are suggested:

a) at low alumina concentrations and high cryolite ratios:

b) at low alumina concentrations and low cryolite ratios:

11. Electrodeposition of Metals from Molten Salts

279

c) at high alumina concentrations and low cryolite ratios:

d) at high alumina concentrations and high cryolite ratios:

With increase in current density and/or decrease in alumina concentration
(in other words, oxygen-containing ionic species), the anode becomes
passivated, leading to discharge of fluoride anions according to the reaction:

and/or

Investigation of the cathodic processes in the electrowinning of
aluminum from alumina-cryolite melts has received far less attention than
the anodic processes. This comes as a consequence of a general opinion that
the cathodic reaction was considered to be simple. However, research in this
field show that cathodic reactions are very complex and the exact
mechanism has not yet been postulated. Cathode reactions in aluminacryolite melts have been studied on molybdenum, platinum, graphite and
molten aluminum electrodes, which has led to significant discrepancies in
the published results. On industrial scale, carbon lining is exclusively used as
a cathode material. Under these conditions, carbon linings exhibit significant
disruption. This disruption is a consequence of the sodium intercalation and
crystal growth of the electrolyte in the carbon. In order to avoid these
problems, composite materials based on
TiC and similar,
have been proposed. The requirements from those materials include physical
properties such as low porosity, high electrical conductivity, excellent
corrosion and chemical resistance under the production conditions and good
wettability by the liquid aluminum. However, proper solutions have not been
found by far, since carbon cathodes are still in use.
At 1000 째C, aluminum is for about 250 mV more positive than sodium.
The sodium ions are principal carriers of the electricity, and as a
consequence, the enrichment of NaF is observed in the vicinity of the
cathode. This causes the appreciable diffusion overvoltage (from 50 to 400
mV, at current densities ca. 0.5 to
The change in the overvoltage

280

Chapter 11

is observed with a change in the cryolite ratio. An increase in the cryolite
ratio, causes a decrease in the cathodic overvoltage. These findings are
related to the laboratory experiments.
In industrial cells, the diffusion overvoltage is significantly lower, due to
convection. Thonstad estimated the cathodic overvoltage at about 100 mV.
Due to enrichment of the cathodic diffusion layer with sodium fluoride, it is
expected that electrowon aluminum metal contain higher concentration of
sodium than the amount corresponding to that of the equilibrium data for the
electrolyte (bulk melt). Most researchers have estimated that an average of
sodium is about 80 ppm, which is below the equilibrium data.
At current densities, which are normally used in the production of
aluminum, the primary gas evolved at the anode is
At lower current
densities
formation of CO in high contents may occur.
Anodic processes in the production of aluminum metal have received far
more attention than the cathodic processes. This comes as a consequence of
the complexity of anodic processes. However, despite the relatively active
research that has gone on the study of these processes, there is no general
agreement among published results nor between explanations of the behavior
observed, which is often associated with poor reproducibility.
For the purpose of studying the anodic reactions involved in aluminacryolite melts, the following electroanalytical procedures have been
investigated: chronoamperometry, chronopotentiometry, cyclic voltammetry,
impedance spectroscopy and related electrochemical methods.8 Materials
used for the study of anodic reactions include various types of carbon,
platinum, gold and refractory metals.
The anodic overvoltage on various types of carbon electrodes in cryolitealumina melts was studied by steady-state measurements. Tafel slopes and
exchange current densities evaluated from these experiments depend on the
nature of the carbon materials. The reported overvoltages are very high. At
overvoltage values are 1.4 V, 1.0 V and 0.8 V for glassy carbon,
graphite and baked carbon, respectively.7 The overvoltage increases with
decreasing porosity, which is attributed to a decrease in the wetted area.
The anodic overvoltage is higher on large anodes due to the shielding
effect of gas bubbles. Dewing and van der Kouwe9, as shown in Figure 11.1
for the graphite ATJ, found Tafel plots with slopes of 0.29.
As this figure shows, the exchange current varies from
for
current densities below
to
for current densities above
The break between
and
is attributed to
changing
ratio in the gas generated. At lower current densities, low
exchange current is due to adsorption of CO on the electrode surface. The
CO, which is produced by reaction of
with dissolved aluminum, acts as
a catalytic poison. The fraction of CO in the gas will depend on how fast
is being generated electrochemically. At higher current densities, more
is produced, which leads to a dilution of CO and to a higher exchange

11. Electrodeposition of Metals from Molten Salts

281

current. For baked carbons the same value of the Tafel slope is obtained,
although it applies over a restricted range of current density and overvoltages
are lower than for graphite anodes.

The chronoamperometric method provides a known and convenient
method for study of electrochemical reactions under diffusion control in
aqueous solutions at room temperatures. Determination of the concentration
of an electroactive species in chronoamperometric experiment is based on
the well-known Cottrell equation:

where I is the time-dependent current,
is the bulk concentration, A is the
surface area, F is the Faradayâ&#x20AC;&#x2122;s constant and D is the diffusion coefficient of
electroactive species. While this approach is well established in aqueous

282

Chapter 11

solutions for analytical purposes under conditions of semi-infinite linear
diffusion to a planar electrode, much less work has been carried out on the
corresponding problem in molten salts at higher temperatures, when many
new serious experimental problems arise.
Anodic behavior in alumina-cryolite melts was studied on carbon, gold
and platinum electrodes by means of chronoamperometry. At the graphite
materials, the anode processes are not fully diffusion controlled, nor are the
results adequately reproducible. At the glassy carbon electrode, anodic
processes are diffusion controlled. This is illustrated in Figure 11.2. As this
figure shows with increasing concentration of
in the melt, the
corresponding current function increases in a satisfactory linear way.

The departure from classically expected behavior at graphite materials is
due to the reduction of the electrode surface in contact with the melt, which

11. Electrodeposition of Metals from Molten Salts

283

is caused by the build-up of evolved adherent gas bubbles or gas films at the
electrode surface.
The results indicate kinetic difference between the anodic reactions of the
graphites and glassy carbon. Evidently, the kinetic reactivity of glassy
carbon for anodic oxidation in the melt is more facile than that for graphite
where their electro-oxidation is not fast enough to become limited by
diffusion control, implying a relative slow electrode process. This difference
could be attributed to the relative stability of the structure of graphite
associated with its multiple conjugated bonding in contrast to that in glassy
carbon, especially on the basal-plane exposures of the graphite crystals of
the materials.
At platinum electrode anodic reactions are limited by the formation of an
oxide film, which obeys, kinetically, Wagner’s parabolic growth law:

where y is the film thickness, K is a constant depending on the diffusion
coefficient oxygen-containing species. For platinum electrode the current
response function does not depend on the
concentration.
To diminish the effect of convection on the electrode-kinetic behavior of
the electrochemical reactions, as well as to minimize disturbances at the
interface due to evolution of gas, the method of fast cyclic voltammetry for
study of the anodic processes in cryolite-alumina melts can be profitably
used. Using very fast cyclic voltammetry researchers found four to five
current peaks in the range 1 to 4 V. The shape and the peak current values in
cyclic voltammograms depend on the kind of carbon material used as the
working electrode in the investigation of anodic reactions in alumina-cryolite
melt.
Typical examples of cyclic voltammograms obtained at different carbon
materials, are presented in Figures 11.3. and 11.4. For the example of
graphite ATJ, four distinguishable anodic current peaks appeared at
approximately 1.1, 1.95, 2.45 and 3.3 V.
In the case of glassy carbon, on the negative going sweep curves of the
voltammogram, between 2 and 3.2 V relatively high anodic currents were
recorded (Figure 11.4.). The sharp decrease in the current at 3.5 V, and a
subsequent absence of current flow on the reverse sweep between 4 and 3.2
V clearly indicate the occurrence of an “anode effect” (see later discussions).
Around 3.2 V, the anode comes out of the “anode effect” permitting current
to flow again. The discussion of the cyclic voltammetry results is restricted
on the second peak at the several kinds of carbon anode materials
investigated. Although the peak current increase with increase of the square
root of the sweep rate for all the kinds of carbon investigated, these

284

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dependencies with the exception of that at glassy carbon, do not show
expected linear relationship. At glassy carbon, however, the dependence of
peak current on the square root of the sweep rate is linear for all
concentrations of alumina. Furthermore, on glassy carbon electrode, the
dependence of the peak current values on alumina content in the range 1.45 â&#x20AC;&#x201C;
4.23 wt.% was observed to follow a satisfactory linear relationship as shown
in Figure 11.5.10

This result indicates that the process is diffusion controlled under these
conditions. At graphite materials the results were not reproducible, nor was
there any observable linear relationship between current maxima values and
concentration. The background current behavior also depends on the
carbon material. In this way a material dependent modification of the
expected ideal current response to alumina concentration is an additional but
unavoidable complication.

11. Electrodeposition of Metals from Molten Salts

285

The AC impedance method has also been applied to and investigated for
the study of the anodic processes in the electrolysis of alumina-cryolite melts
at carbon.11 This method is used to investigate the discharge of the
oxyfluoroalumnate ion at a graphite electrode. An increase of the applied
anodic overvoltage leads to a variation of the shape of the complex-plane
impedance diagram. For zero overvoltage with a residual alumina
concentration of 0.46 wt.%, the plot of imaginary part of the impedance (Z”)
versus the real part of the impedance (Z’) follows a linear relationship
having a 45 ° slope, which is recognized as diffusion impedance. With
further increase of the overvoltage, the plots of Z’ versus Z” show inflections
of the straight line and these are more pronounced as the overvoltages
applied to the electrode are increased. An inductive loop appears at low
frequencies. The change in the shape of the Z’ versus Z” plot with increase
of the overvoltage was explained by the influence of reactions producing
bubbles of gaseous compounds of oxygen with carbon which disturb and
reduce the thickness of the diffusion layer.

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In spite of the intensive studies of alumina-cryolite melts by various
electrochemical techniques, significant disagreements among results
appeared in the published literature. These problems are consequences of
nonreproducibility of results which arise due to the nature of the carbon
sensor electrode materials used and, also due to a number of technical
problems including the presence of dissolved aluminum metal in realistic
practical systems.

11.3

THE ANODE EFFECT

The anode effect is a phenomenon that has been observed in many
processes involving the electrolysis of molten salt. It is described as a
blockage effect, which inhibits the current flow between the anode and the
melt. Due to gas evolution, growth of bubbles and their coalescence occur
covering most if not entire surface of the anode. In industrial cells during
electrolysis of alumina-cryolite melt the anode effect manifests itself through

11. Electrodeposition of Metals from Molten Salts

287

an immediate increase of cell voltage from values between 4.1 and 4.3 V,
during normal electrolysis to about 35 to 60 V, and sometimes even up to
130 V, depending on the current density.7 The cell remains under the
influence of the anode effect until the current is interrupted, which allows
adherent gas bubbles formed at the anode surface to collapse or become
detached. The effect is somewhat analogous to that observed in anodic
evolution at carbon from KF 2HF melts in commercial cell operation.
The reasons for appearance of the anode effect are not yet established.
Chemical analysis of the anode gases shows that they contain up to 30 %
fluorine compounds such as
and
The presence of fluorocarbon
compound promotes dewetting of the anode surface and the growth of large
bubbles. In industrial cells the anode effect arises when the alumina
concentration in the melt is between 0.5 and 2 %. Thus, maintaining good
control of alumina content is very important factor in avoiding the anode
effect. Upon an occurrence of the anode effect, the crust on the top of the
melt is broken and alumina is added.
Conditions for onset of the anode effect are associated mainly with the
depletion of alumina concentration in the melt during electrolysis, increasing
potential, and presence of fluorocarbon surface compounds at the carbon
anode surface, causing dewetting of the anode by the electrolyte and
adherence of gas bubbles.12

11.4

NONCONSUMABLE ANODE MATERIALS IN
MOLTEN SALTS ELECTRODEPOSITION

In the electrowinning of metals from their molten salts, due to significant
corrosion at higher temperatures and anodic reactions involved in the process,
the graphite or other anode materials are often very easily consumed.
The search for nonconsumable, or inert anodes, has been an important
research activity for a long time, especially in the electrolysis of aluminacryolite melts, which will mostly be discussed here, although other molten
salts, depending on their composition and conditions have attracted
considerable attention. The requirements for materials, which could be used
as anodes for in alumina-cryolte melts include resistances to attack by
molten cryolite and oxygen, high electronic conductivity, mechanical
strength and resistance to thermal shock. The possibility of using
nonconsumable anodes in the electrowinning of alumina has become
attractive for the following reasons:
(i)
(ii)

These anodes would not be consumed during electrolysis
The oxygen which would be formed at the anode could be utilized
industrially

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Chapter 11

(iii)

The problems related to contamination of the working environment,
when the Hall-Héoult process is used could be reduced
The corresponding cell design would permit electrolysis with higher
current efficiencies than is currently possible with carbon anodes.
All above-mentioned factors could represent significant savings to
the aluminum production industry.

(iv)
(v)

In the case of nonconsumable anodes, the production of aluminum would
be represented formally by the equation:

(E°=-2.19 V at l010°C)
In the search for an inert anode for use in the Hall-Héroult electrolysis,
among many accessible materials, oxides, metals, refractory hard metals and
gaseous fuel anodes have been investigated.13
Among the oxides, investigated as materials for anodes in electrolysis of
an alumina-cryolite melt should be mentioned the cold pressed and sintered
anodes of
NiO, CuO and
the ferrites such as
and
stabilized
as a possible inert material for
production of less corrosion resistant anodes, the
anodes and
complex anodes
The Cu-containing cermets
spinel, NiO and metallic phase
which is mostly Cu) have also been investigated as possible anode materials
for the primary aluminum industry.
In the case of gaseous-fuel anodes, the production of aluminum metal is
described by the following reactions:

The hard refractory materials such borides, carbides and nitrides of the
transition metals such as
mixtures,
MoSi and TiCr
should be mentioned.
Metal anodes such as copper, nickel, chromium, tungsten stainless steel
and silver are unresistant in alumina-cryolite melts. On platinum and gold,
formation of oxide films and/or corrosion occurs.
It seems that oxide materials (nickel ferrites type) are the most promising
by far.14 However, as research shows the solubility of alumina in cryolite

11. Electrodeposition of Metals from Molten Salts

289

melts is dependent on the content of alumina. In order to exhibit a slow
dissolution of oxide anodes, the alumina concentration should be maintained
at a relatively high level. The alumina content in the industrial cells with
graphite anodes is maintained at 2 to 4 %. If the oxide type of anodes are to
be used in the production, the content of alumina in the melt should probably
be kept at higher levels, which should be determined by the additional
studies.

The metal processing industry produces various toxic gases and aqueous
effluents containing ions of heavy metals, or in some case cyanides. Most of
these metals are toxic. Regulations of the Environmental Protection Agency
(EPA) require specific control of all air pollutants and hazardous waste.
Areas of the environmental management in the electrometallurgy field
involve air pollutants, and other waste treatment and disposal. In order to
minimize the negative impacts of industries involving electrowinning,
electrorefinery and plating technologies to the ecosystem, an adequate
treatment of environmental should carefully be taken into consideration.
Safety management in the electrometallurgy is directed towards electrical
hazards, explosion hazards and hazards arising due to handling and exposure
to dangerous chemicals.
The electrical hazards arise from the fact that the electrometallurgical
plants and plating shops operate with both direct and alternating currents.
Most cells are designed in a way to minimize potential difference from the
ground potential. In some plants (i.e., aluminum electrowinning from aluminacryolite melts) strong magnetic fields are generated in cell rooms. The
regulations prohibit magnetic and conductive materials in the cell rooms.
The explosion hazards depend on the system, but are often associated
with plants in which, hydrogen and chlorine evolution reactions are involved
in the process. Other explosion hazards may include chemicals, i.e. reactive
metals such as alkali metals, magnesium etc. The regulations in this case
require proper safety equipment (e.g., glasses, masks, aprons etc.).
Discussion in this chapter is divided into following sections:
(i)

ENVIRONMENTAL CONCERNS IN THE
ELECTROWINNING AND ELECTROREFINING
FROM AQUEOUS SOLUTIONS

Metals produced by the electrolysis from aqueous solutions include Cd,
Cr, Cu, Mn, Co, Ni and Zn. Most of these metals are electrowon from
solutions containing sulfuric acid, although metals such as cobalt and nickel
can also be produced from chloride-type acidic electrolytes. Metals are
deposited on cathodes, while anodic reactions, depending on the anions
present in the electrolyte, usually include oxygen (sulfate solutions) or
chlorine (chloride solutions).
The evolution of oxygen in the case of sulfate electrolytes is
accompanied by lowering of the pH due to sulfuric acid formation.
Consequently, an electrowinning process, which is carried out from a sulfate
solution, results in the generation of sulfuric acid. In the case of
electrowinning from chloride electrolytes, chlorine is generated at the anode
and removed through suitable hoods in order to avoid environmentally
objectionable fuming.
The environmental issues in the electrowinning technologies include the
proper treatment of waste solutions. These solutions contain significant
amounts of sulfuric acid in addition to metal that is electrowon, and traces of
other heavy metals, which may have a negative impact on the environment.
After the electrolysis the sulfuric acid (e.g. processes involving
electrowinning or electrorefining of Cu, Zn or Ni) is usually recovered and
concentrated from residual solutions. This sulfuric acid is reused in the
process. The residual solution, after the electrowinning or electrorefining is
treated in combustion evaporators, and about 70 % of acid is recovered for
recycle. The evaporators perform quite satisfactorily; however,
disadvantages include high-energy and high-maintenance costs. Ionexchange resins, in which ions are adsorbed, are also considered, however,
these systems involve large amounts of water and the recovered acid is
therefore weaker than acid in the feed material.
Another process for recovery and concentration of sulfuric acid includes
electrodialysis technology.1,2 In electrodialysis technology, the ion-exchange
membranes are used for the separation process. The ions selectively
permeate exchange membranes by migrating from one site to the next under
the influence of electrical current. The ion-exchange membranes are
arranged into continuous operation and do not require periodic stripping as
with ion-exchange resins. Membranes must have a high degree of
permselectivity (a selective permeability to a specific type of ion).
Electrodialysis systems with monovalent anionic and cationic permselective
membranes are used for the removal of monovalent ions such as chlorides,

12. Environmental Issues

293

fluorides, sodium, potassium etc. The monovalent permselective membranes
retain divalent ions (e.g.
etc.) in the dilute or electrolyte
stream. The results showed a very good recovery of sulfuric acid from an
acidic nickel sulfate stream by electrodialysis, where more than 80 % of the
acid were recovered.
For the removal of heavy metals from residual solutions, chemical,
biological, or electrochemical methods are applied.3-6 The products or
removal are either returned back in the electrowinning process or sold as
various salts.

12.2

ENVIRONMENTAL CONCERNS IN THE
MOLTEN SALTS ELECTROLYSIS

Environmental concerns in molten salt electrolysis (e.g. electrowinning
of aluminum, magnesium etc.) depend on the metal and production
conditions. In aluminum production pollution problems arise due to cell
design and operation. The following facts are of great concern:
i)
ii)
iii)

Fluoride emission
Hydrocarbon fumes, and
Dusting problems

Sources of fluoride emission include the gaseous compounds generated at
the anodes and melt vapor pressure (i.e.,
Due to presence of
moisture, fluorides hydrolyze, forming the HF gas. The modern aluminum
plants utilize cells where the gases produced during electrolysis are collected
and adequately scrubbed. Dry scrubbers are used in the treatment to catch
particulates and adsorb HF on alumina, and fed back into the cell.
The hydrocarbon fumes originate from anode baking and they are
generally disposed by burning. The emission of highly polluting gases
during the manufacture of the carbon anodes and cathodes is carefully
controlled. Dusting problems due to alumina handling are usually solved
with hoods and exhaust systems, which collect the dust. The dust is
separated by cyclones or filter bags etc. and recycled to the process.
The spent linings are the largest volume of waste in the production of
aluminum. In order to prevent contamination of the environment, these
waste materials should be properly stored to avoid leaching of toxic
constituents such as cyanides and fluorides. The research is directed toward
recovering valuable components from the waste and destroying the cyanide.
The recovered materials are recommended for use in the manufacture of
cement or mineral wool.

294

Chapter 12

Other toxic constituents in the aluminum electrowinning industry include
polyaromatic hydrocarbons, sulfur dioxide and hydrogen fluoride.
Polyaromatic hydrocarbons are formed in industrial cells during the baking
process.7 These compounds are known as carcinogenic agents.
In addition to alumina - cryolite melts other molten salts electrolysis
systems represent important health hazard requiring significant safety
precautions. These systems include alkali metals e.g., lithium, sodium and
potassium, magnesium etc. Inappropriate handling of alkali metals can result
in serious injuries such as burns, blindness and even fatalities. The hazards
arise due to the tendency of alkali metals to vigorously react with water, or
to oxidize in air. In the reaction of sodium with water, hydrogen, sodium
hydroxide and significant amount of heat are produced. This combination
results in the explosion when air is present. The common fire extinguishers,
such as water,
and
should never be used, since all these
compounds react with alkali metals. Pure metallic sodium is usually stored
under organic solvents. In working with sodium or other alkali metals, face
shields, hard hat, hoods and multiple layers of flame-retardant protective
clothing are recommended.
In the plants where production of metals is carried out from molten
chlorides, the main anodic reaction is the chlorine evolution reaction.
Production and utilization of chlorine is considered safer than its
transportation. The chlorine is usually collected and used in different
industrial processes. Major processes utilize chlorine and create hydrogen
chloride as a by-product, which is used in industrial fields. Chlorine is
produced as the main anodic product in the electrolysis of fused
It is
either recycled in the process or sold commercially. Chlorine is stored and
transported as a liquefied gas in cylinders under pressure. Exposure to
chlorine causes irritation of the eyes and the mucous membrane of the
respiratory tract.
for humans in 30 minutes is estimated at about 840
ppm, while amounts causing severe symptoms in 30 to 60 minutes are
estimated at 40 to 60 ppm.

12.3

ENVIRONMENTAL CONCERNS IN THE
ELECTROPLATING TECHNOLOGIES

In the electroplating technologies safety concerns are directed towards
the handling and exposure to chemicals and solutions of various toxicity and
waste treatment. A typical electroplating technology involves steps, which
are outlined in the block diagram presented in Figure 12.1. As this block
diagram shows, a typical electrodeposition technology includes steps such as
degreasing, pickling, cleaning and electroplating. It is worth noting that
similar types of operations and environmental concerns are involved in the

12. Environmental Issues

295

electroless plating. Some of these steps are repeated several times. and order
of these operations (steps) can be changed, depending on the substrate and
on the metal.

In order to remove residual chemicals on the surface of the substrate
between these steps, rinsing with water is involved. Rinsing minimizes
contamination, however, it increases the volume of waste solutions. The last
rinse, prior to drying is very important since any traces of residues can lead
to staining and even corrosion. This rinsing is carried out with hot deionized
water, and sometimes with methyl/ ethyl alcohol, for faster drying. For
efficient rinsing, spraying with water is very useful, and environmentally
very important since it reduces the volume of waste solutions. Depending on
the metal or alloy being plated, and on the nature of the substrate, operations
in the electroplating technology include different solutions and substances,
which are more, or less toxic. It is generally accepted that most of the
substances used in this technology are dangerous up to various degrees.
The waste solutions in the electroplating technologies are divided into the
following types:

Whenever possible preventive measures should be taken in order to minimize the volume of waste solutions. This is usually achieved with the introduction of additional equipment (e.g., tanks) as well as prolonged time of some operations (e.g., rinsing). Obviously, the preventive measures require
extra investment, although this may lead to a very successful treatment.
Treatment of waste solutions is usually carried out by chemical or physical
methods.
The
degreasing
solvents
(chloroform,
methylene
chloride,
tetrachlorethylene, carbon tetrachloride etc.) are often inflammable,
however, they are sources of harmful gases. In this operation, oils, separated
from substrates can produce suspended and deposited solids, which may
require the introduction of an additional step (e.g., filtration) in order to
separate them from degreasing solvents. Emulsified oils are separated with
aluminum sulfate, at a pH of about 5 to 6, while solutions are agitated with
bubbled air. The scum formed at the surface is removed, and residual
solution treated with other waste solutions.
Cleaning processes with biological separation are also used in the electroplating and surface finishing industry.8 These processes operate at a low
temperature and low pH and are applied on metals such as Al, Zn, Cu and
Fe. The advantage of the cleaning processes with biological separation is
that metals are not etched. Solutions used in the degreasing process are
slightly alkaline and contain bacteria, which destroys the oil that exists on
the metallic parts. Decomposed oil particles and bacteria are continuously
separated from the cleaning solution as sludge in significantly smaller
volumes than the associated with traditional cleaning methods.
Pickling solutions are acidic and they contain metal ions. The most
commonly used reagents for the pickling process are hydrochloric and
sulfuric acids. Acids containing fluorides are used for a pre-treatment of
silicon containing alloys. These acids produce corrosive fumes and adequate
ventilation is required. In order to protect the environment a proper disposal
of waste pickling solutions is applied in the industry. Acidic waste solutions
used in the pickling or non-cyanide waste acidic solutions used for zinc,
nickel and copper plating are removed into separate tank or in a tank with
alkaline waste solutions. In this step, the neutralization of solutions may take
place, although, with an addition of lime, sodium carbonate or sodium
hydroxide, the heavy metals are precipitated at pH 8.0 to 8.5. The amount of

12. Environmental Issues

297

required chemicals is determined on the basis of chemical analysis of metal
content (e.g. Cu, Ni, Fe, Cd, Zn, Cr etc.) in the waste solutions. After the
precipitation, suspensions are filtered to separate the sludge and dispose the
water.
Plating solutions may contain cyanides, chromates, heavy metals (copper,
cadmium, lead, zinc and nickel). Most of these substances are toxic and also
carcinogenic. Among substances used in plating technologies the following
substances are considered as toxic and/or carcinogenic: cadmium, chromium, nickel, lead, mercury, cyanide and their compounds, organic solvents
such as benzene, carbon tetrachloride, chloroform, toluene, xylene etc.
Significant amount of research is funded to investigate proper technologies
for the treatment of waste solutions, or to replace some of processes
involving cyanides, chromates etc., with alternative, environmentally
friendlier technologies. Among these alternatives, cyanide free solutions for
gold, silver or copper electroplating should be mentioned. Also, research
towards replacement hard chromium9 and cadmium plating is carried out.
Substitutes for hard chromium plating, up to certain levels include Ni-Mo,
Ni-W and Ni-P plating. However these technologies involve substances,
which are on the list of cancerogenic or toxic chemicals. On the other hand,
physico-chemical properties of alternative coatings are frequently not
comparable to those of hard chromium. For the replacement of cadmium
substitutes are being sought in alternatives such as zinc, or Ni-Zn, or Sn-Zn
alloys. However more research is required until cadmium plating will fully
be replaced with other solutions.
Cyanides are well known as true non-cumulative protoplasmic toxic
compounds. They react with enzymes of the blood that regulate oxygen
transfer to cellular tissues. Exposure to cyanide solution causes severe
complications and even death. The toxicity of cyanide solutions is a result of
the free cyanide ion,
or HCN. Hydrogen cyanide can enter the body by
inhalation, oral ingestion or skin absorption.
Hydrogen cyanide undergoes an exothermic polymerization (at pH 5 to
11, especially in the presence of water or heat). This reaction can become
explosively violent.
Work with cyanide solutions is carried out in a extremely well ventilated
fume hood, and with special safety equipment, which includes air-masks,
face-masks, rubber gloves, plastic aprons etc..
Cyanide solutions are used in plating of Cu, Zn, Au, Ag etc. Small
amounts of cyanide solutions can be decontaminated in reactions with
sodium hydroxide (pH > 12) and then with ferrous sulfate. The resulting
ferrocyanide is relatively non-toxic.
Disposal of waste cyanide solutions is carried out with chlorine or
sodium hypochlorite. With larger quantities, such as industrial waste cyanide

298

Chapter 12

solutions, treatment is carried out in alkaline media, according to the
following reactions:

The less toxic sodium cyanate, NaCNO, is further destroyed with
sufficient amount of chlorine:

In this process, the pH is maintained above 10 in order to avoid formation
of nitrogen trichloride or cyanogen chloride.
With smaller volumes, sodium hypochlorite is used instead of chlorine.
Other methods for destruction of cyanides include hydrogen peroxide, ozone,
permanganate, biological decomposition10, removal by ion exchange and
recovery, lime-sulfur reaction to give sulfate, electrochemical methods11 etc.
Chromium plating is widely used for different engineering and decorative
applications. However, compounds used in chromium plating (especially
those of Cr(VI)) are recognized as very toxic and cancerogenic. Consequently, at a working place the regulations require safety glasses, rubber gloves
and plastic aprons.
The treatment of waste solutions from chromium plating is based on a
two-stage process. In the first, the Cr(VI) is reduced to Cr(III) and in second
stage, the Cr(III) is precipitated as hydroxide and disposed as sludge. The
reduction of Cr(VI) is carried out in the acidic medium (pH<3), by adding
sulfuric acid. Several compounds are used as reducing agents of Cr(VI),
species. These reducing agents include sodium bisulfate, sodium thiosulfate,
sulfur dioxide, aluminum powder etc. After completed reduction, the waste
solution containing Cr(III) species is collected for the precipitation. The
precipitation is carried out with hydrated lime or sodium carbonate. The
settlement of the sludge is improved by adding aluminum sulfate, ferric
chloride etc. After the settlement, the sludge is separated (e.g. by decanting)
and disposed. The sludge should not contain oxidizing agents (e.g.
manganese dioxide, hypochlorite, chlorine or some organic oxidizing
substances). These compounds can convert Cr(III) to Cr(VI). Recoveries of
Cr(VI) compounds or as chromium metal are also recommended.
Physico-chemical methods of treatment of residual solution from
electroplating process include evaporation, ion exchange, reverse osmosis,
electrodialysis and electrolytic metal recovery.
The current status shows that in developed nations significant attention is
paid towards researching and finding solutions related to the environmental
concerns in the electrometallurgy field. It is expected that this research and

12. Environmental Issues

299

investment in improvement of safety standards will continue to develop in
the future for the safe and healthy environment in rapidly growing modern
technology. It is certain that electrometallurgy related technologies will
change as new information is uncovered. Adapting to these changes in terms
of pollution prevention can be accomplished in an analytically sound
manner.